Photo libraries grow faster than expected because the biggest items are not photos. A few minutes of high-resolution video can outweigh a thousand stills, and burst mode saves dozens of near-identical frames.
Cloud photo services then multiply the problem by keeping a full local copy on every device unless optimised storage is enabled.
Deleting rarely frees space immediately either — items sit in a recently-deleted album for a month by design, which is helpful until you are trying to install an update.
A twenty-minute pass over videos, bursts and screenshots typically recovers more space than weeks of careful photo-by-photo pruning.