At the end of this script that imports MXDs into an ArcGIS Pro Project I'm using the saveACopy function.
<SPAN class="string token">'''Import folder of MXD(s) into an ArcGIS Pro Project and save the project in a folder.
'''</SPAN>
<SPAN class="keyword token">import</SPAN> arcpy<SPAN class="punctuation token">,</SPAN> os
<SPAN class="comment token">#List full path location of MXD(s) in workspaceMXD </SPAN>
workspaceMXD <SPAN class="operator token">=</SPAN> r<SPAN class="string token">"C:\gisfile\GISmaps\AtlasMaps\ATLAS_MAPS_20\TestFolder"</SPAN>
arcpy<SPAN class="punctuation token">.</SPAN>env<SPAN class="punctuation token">.</SPAN>workspace <SPAN class="operator token">=</SPAN> workspaceMXD
mxdlist <SPAN class="operator token">=</SPAN> arcpy<SPAN class="punctuation token">.</SPAN>ListFiles<SPAN class="punctuation token">(</SPAN><SPAN class="string token">"*.mxd"</SPAN><SPAN class="punctuation token">)</SPAN>
<SPAN class="comment token">#aprx project that MXD(s) are being imported into</SPAN>
aprx <SPAN class="operator token">=</SPAN> arcpy<SPAN class="punctuation token">.</SPAN>mp<SPAN class="punctuation token">.</SPAN>ArcGISProject<SPAN class="punctuation token">(</SPAN>r<SPAN class="string token">"C:\gisfile\GISmaps\AtlasMaps\MXDtoAPRX_tempdelete\MXDtoAPRX.aprx"</SPAN><SPAN class="punctuation token">)</SPAN>
<SPAN class="keyword token">for</SPAN> mxd <SPAN class="keyword token">in</SPAN> mxdlist<SPAN class="punctuation token">:</SPAN>
arcpy<SPAN class="punctuation token">.</SPAN>env<SPAN class="punctuation token">.</SPAN>overwriteOutput <SPAN class="operator token">=</SPAN> <SPAN class="token boolean">True</SPAN>
fullpath <SPAN class="operator token">=</SPAN> os<SPAN class="punctuation token">.</SPAN>path<SPAN class="punctuation token">.</SPAN>join<SPAN class="punctuation token">(</SPAN>workspaceMXD<SPAN class="punctuation token">,</SPAN> mxd<SPAN class="punctuation token">)</SPAN>
<SPAN class="keyword token">print</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="string token">"Importing: {}"</SPAN><SPAN class="punctuation token">.</SPAN>format<SPAN class="punctuation token">(</SPAN>fullpath<SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN>
aprx<SPAN class="punctuation token">.</SPAN>importDocument<SPAN class="punctuation token">(</SPAN>fullpath<SPAN class="punctuation token">)</SPAN> <SPAN class="comment token">#import the MXD(s) into Pro</SPAN>
<SPAN class="comment token">#the count of the maps</SPAN>
<SPAN class="keyword token">print</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="string token">"Currently {0} maps in Project file."</SPAN><SPAN class="punctuation token">.</SPAN>format<SPAN class="punctuation token">(</SPAN>len<SPAN class="punctuation token">(</SPAN>aprx<SPAN class="punctuation token">.</SPAN>listMaps<SPAN class="punctuation token">(</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN>
<SPAN class="comment token">#name the maps</SPAN>
aprx<SPAN class="punctuation token">.</SPAN>listMaps<SPAN class="punctuation token">(</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">[</SPAN><SPAN class="operator token">-</SPAN><SPAN class="number token">1</SPAN><SPAN class="punctuation token">]</SPAN><SPAN class="punctuation token">.</SPAN>name <SPAN class="operator token">=</SPAN> mxd<SPAN class="punctuation token">.</SPAN>split<SPAN class="punctuation token">(</SPAN><SPAN class="string token">"\\"</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">[</SPAN><SPAN class="operator token">-</SPAN><SPAN class="number token">1</SPAN><SPAN class="punctuation token">]</SPAN><SPAN class="punctuation token">[</SPAN><SPAN class="punctuation token">:</SPAN><SPAN class="operator token">-</SPAN><SPAN class="number token">4</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="comment token">#save a copy of the project in the workspace folder</SPAN>
aprx<SPAN class="punctuation token">.</SPAN>saveACopy<SPAN class="punctuation token">(</SPAN>fullpath<SPAN class="punctuation token">)</SPAN>
<SPAN class="keyword token">print</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="string token">"----done----"</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="line-numbers-rows"><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN></SPAN>Output
Importing<SPAN class="punctuation token">:</SPAN> C<SPAN class="punctuation token">:</SPAN>\GISmaps\AtlasMaps\ATLAS_MAPS_20\TestFolder\Union_Elem_81_B<SPAN class="punctuation token">.</SPAN>mxd
Currently <SPAN class="number token">2</SPAN> maps <SPAN class="keyword token">in</SPAN> Project file<SPAN class="punctuation token">.</SPAN>
Importing<SPAN class="punctuation token">:</SPAN> C<SPAN class="punctuation token">:</SPAN>\gisfile\GISmaps\AtlasMaps\ATLAS_MAPS_20\TestFolder\ValleyView_Unit_365U_B<SPAN class="punctuation token">.</SPAN>mxd
Currently <SPAN class="number token">4</SPAN> maps <SPAN class="keyword token">in</SPAN> Project file<SPAN class="punctuation token">.</SPAN>
Importing<SPAN class="punctuation token">:</SPAN> C<SPAN class="punctuation token">:</SPAN>\gisfile\GISmaps\AtlasMaps\ATLAS_MAPS_20\TestFolder\WillCounty_Elem_92_B<SPAN class="punctuation token">.</SPAN>mxd
Currently <SPAN class="number token">6</SPAN> maps <SPAN class="keyword token">in</SPAN> Project file<SPAN class="punctuation token">.</SPAN>
Importing<SPAN class="punctuation token">:</SPAN> C<SPAN class="punctuation token">:</SPAN>\gisfile\GISmaps\AtlasMaps\ATLAS_MAPS_20\TestFolder\Wilmington_Unit_209U_B<SPAN class="punctuation token">.</SPAN>mxd
Currently <SPAN class="number token">8</SPAN> maps <SPAN class="keyword token">in</SPAN> Project file<SPAN class="punctuation token">.</SPAN>
<SPAN class="operator token">-</SPAN><SPAN class="operator token">-</SPAN><SPAN class="operator token">-</SPAN><SPAN class="operator token">-</SPAN>done<SPAN class="operator token">-</SPAN><SPAN class="operator token">-</SPAN><SPAN class="operator token">-</SPAN><SPAN class="operator token">-</SPAN><SPAN class="line-numbers-rows"><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN></SPAN>
It saves an APRX file from the workspace folder for each imported MXD. But, a copy of the map is imported in each consecutive project. So, as seen here and in the Test Folder, by the fourth saved APRX file maps for the other three projects are also there.

That's why the file size of the APRXs increases by 400+ KB as the script loops through the folder.

I've also tried taking aprx.saveACopy(fullpath) out of the for loop. But, that only saves one APRX file with all maps in that one project.
How do I loop through the Test Folder and save a copy of the APRX file with it's corresponding map only?
As discussed at the bottom of my other post, ArcGISProject saveACopy seems only to allow full paths as a parameter.
importDocument() OSError