Your uploaded observations are not connected to original photos on your phone, but app can get a lot of size (look up https://forum.inaturalist.org/t/limit-cache-size-or-allow-cache-clearing-for-apps/2059). You can even delete original photos, but be careful as iNat saves smaller size files than what your camera captures and there could be errors potentially resulting in loss of data, so you may want to keep them somewhere.
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