PDF files are not linear. The fact that a 100-page file is 10 MB does not mean that a 50-page file will be 5 MB.
One way you can try to reduce the file size is to do a Save As on the new files and save them under a new name.
You can also try to use File - Save As Other - Reduced Size PDF command, but there's no guarantee it will work.
Thank you everyone for your input on this question. Actually, I was able to reduce the size of the files by using the "optimize" tool. 7 pages at 14 mb were thus reduced to 1.4 mb.