When you install from play store the apk is installed automatically. When you download from third party you have to install yourself. Apk version isn't a thing. I wouldn't download from third party if it is available on the play store. That would be opening the possibility for malware for no reason.
At the very least it would take up the same amount of space, it could certainly take up more space if it's been modified to include malware along with it. Considering apk/installer space =/= installed space, comparing apk size itself is not a good metric to use.
Use APK mirror its been safest over the years ive used it , if its same apk app that you would get from play store should be same , if you install different version of the same app then could be different size