I use Tapatalk, the forum-browsing app, on both my iPhone and iPad to browse Forums (including this one!)
Tapatalk has a "Clear Cache" option in its settings that allows a user to clear the app's cache (mostly cached photos, videos and other similar media) and that will inevitably reduce the "size" of the app right down again.
So it is clearly possible on an iOS device, but just down to the relevant app's Developer to implement?