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.

No. Unfortunately not. But it seems to disappear, when i uncheck Settings > Appearance > Cards > Full height images. For some reason, i can now enable the same setting again, without that the milky images comes back. On the other hand, it doesn’t change the height of the images.