Most space used by games is taken up by textures and audio. Textures can be downscaled (halving texture resolution reduces the size by 75%) and audio can be resampled at a lower bitrate without the quality drop being too noticeable on tiny phone screens/speakers.
Maps, warframes, weapons and skins will also likely be loaded as needed, so you'll probably start with 20GB but bloat to 60 as you play
Another trick I noticed playing CoD mobile is that they don't load every other player's skin, only the latest. So if your install doesn't have it, it only display's the default skin for other players. Which also mean that the skin you paid 60$ for in the last Season Pass is only visible by a few players.
Really? Looking online, it seems to be taking ~30GB, which isn't much relatively speaking (considering modern AAA games these days can be 100+GB).
Phones nowadays come with 128GB as base option, and typically even offer a 256GB model (eg the Pixel 8). So Warframe shouldn't really be an issue for mobile gamers who typically buy beefier phones than the average.