My experiences for the file size:
-large images and/or pdf greatly increase the size. AC store the original image in pln
-layout book with detailed drawings could increase size as well (especially using consultant DWGs)
If you want to find the problematic area try :
-delete all images from project and save. Watch the size after save
-publish all the placed drawings to PMK and se the size of PMKs
Anyway don't bother about file size, my biggest pln size is 1.7GB (without compression) and works well.
i7-4770K / 16GB / GTX560 2GB / 512GB SSD / Win10 64bit
AC21 (HUN-64 bit) / Bimx Pro / Revit 2017