Hello!
This is the intended functionality for adding a background to a legend. Legends are dynamic, growing and shrinking as new layers are added to the map. The background is designed to grow and shrink with the legend, so that it works well with all legend sizes as well as the different legend fitting strategies and legends in map series. There are easy ways to get the effect you are looking for though!
If you need the box surrounding your legend to be an exact size, you can draw a rectangle and place it behind your legend, you could then group the two elements together (Select the legend and rectangle in the Table of Contents, right click -> Group) so you can resize or re-position them both at the same time. This will give you control over the exact size and shape.
If you just want the background to extend beyond the legend items a little, creating a margin, you can use the X-Gap and Y-Gap controls, located just under the Background Symbol. Increasing these gaps creates a set margin around your legend. This allows your background to still grow and shrink as you add or remove legend items.
Cheers,
Aubri Kinghorn
Cheers,
Aubri