It is to do with how the app “caches” observations on your phone. The list of observations shown is generated from when iNat app connects to the network and “refreshes” the list (saving a copy of every observation involved). But the app also seems to save a copy of any observation you upload, as you will notice the “documents & data” size will rise according to the size of the photos you had uploaded with observations.
Logging out and back in will clear the cache and lower the app size back to its usual amount.