You are probably caching your maps on the device.
It can be turned off in the settings, however you'll use more network data then.
I have it turned off and the app only uses 119kb of extra data for me.
Don't think that is it. There is something else going on. I catch my maps on the device, but wz maps are small (like not close to GB's. Maps have a maximum size to be approved as well.) and the app should also free up map storage if unused or needed.
I have map catch ON and app size is 118.5MB + Documents and Data 17.9MB.
Either you found a crazy memory leak or something or your phone is broken? Just reinstalling the app should fix things thru.