Monika,
I really doubt this 33x difference is caused by just the difference between ArcMap and Pro. In my experience, with both pure 100% vector maps, and mixed raster & vector maps, the difference between ArcMap and Pro PDF file size output is more likely to be in the 2-3x range max.
I therefor recommend you to carefully review your export settings. E.g. if your map contains raster data, changing the output dpi from e.g. 300dpi to 600dpi, will already cause an approximately 4x times bigger file size. This is unrelated to the difference between ArcMap and Pro, but purely because of a doubled raster image resolution in the PDF output.
Considering you have a 33x times larger file size, suggests you may be using a dpi export value about 5,75x bigger, e.g. instead of 300dpi, something like 1800dpi.
Generally speaking, you should never need a higher value than maybe 600dpi. Even at 300dpi, print quality should be acceptable for most applications. At 600dpi, it should satisfy even the most demanding printing processes. This is especially true for Pro, as text halos are better handled than in ArcMap. In ArcMap, with lower dpi values, text halos (e.g. white background behind black text) may be of lesser quality, as the halos are not stored as text, but as vector graphics using a vector coordinate snapping grid equal to the dpi value. The higher the dpi, the smoother and more accurate the text halos become in ArcMap. This is not the case in Pro as far as I can tell from reviewing maps. Pro will output good quality text halos even at the relatively low recommended base dpi setting of 300dpi.
Marco
