I was about to install PixelFed, listed in Google Play as a 13-MB app, but it demanded to delete apps before I free 500 MB. Similar thing happens with F-Droid, although it gives less data. Why? Does it not, like Flatpak on PC, list dependencies? But that’s dumb; plus, no dependency for a browsing app may take 500 MB of space, and the demands aren’t getting smaller when more apps are on device.
13MB is the download size in GP. This is different from fDroid, because you download only a device or architecture specific installer, which is different on fdroid. After installing this extracts to maybe 75mb, which should be identical to fdroid. After first start the app, this can download additional packages, if needed and the wasted space grows. It grows more with the cache and other temporary content. A app can also reserve some space. For some apps, this could be up to 5gb and more.