It depends on the app, but the amount downloaded doesn't mean that's how big the app will be on the device, there'll be some compression done to make the download smaller, plus once the downloaded file itself runs the resulting app is usually a bit bigger anyway.
If you've got 64GB app space should not be a worry, in general it's video that takes up most room by some way as the files for that are relatively huge.
(to check use the method above)