The differences in SIZE are probably the difference between a Megabyte and a Megabyte. The distinction comes from the fact that one may be using powers of 2, and the other, powers of 10.
210 = 1024 "1 kb"
103 = 1000 "1 kb"
So you see there is a difference of "24". Small difference, but enough to notice.
Note that there has been some attempt to distinguish using strange units like "kibibits", which really haven't taken on, therefore powers of two are generally represented by the nearest power of ten and you have a bit of difference.
If you're worried about the security of those applications, then you should proceed to UNINSTALL them. That's about all you need to do to regain your security.
And also note that just because something is distributed to you through a distribution channel that you trust, does not mean that the application is SAFE. Playstore does NOT provide you with any means to verify that an application you installed from there is actually secure. There could be any number of obfuscated security holes in any precompiled application you happen to find and nothing will tell you that its there. The ONLY thing you really have to go on is whether or not you TRUST the vendor.
F-Droid, on the other hand, provides you with a copy of the exact source code used to compile every version of every package they distribute! So if you're worried about something being slipped in, just go through the code!