Mac App Store version vs GOG.com version

[amfek]

I already own the GOG version so I'm not going to get the MAS version. I can see the file size of 21gb in Finder so that's the installed size. MAS app sizes are actual installed sizes too as far as I know so that still leaves me wondering what might account for the variance. The MAS version is also named differently (SOMA 2015). Not sure if that means anything.