Made an issue here after talking to our Android developer: https://github.com/inaturalist/iNaturalistAndroid/issues/1028
If the photo is still on the device (so for situations where a) you’re making a new observation or b) editing a synced observation made from your device) we should be able to edit that full-sized photo. If the photos are from an observation made via the web or another device or they no longer exist on your device, that’s not possible because we only have access to the resized photo.