If the point of a Google Photos app is to access / back up your photos via Google Photos (web), then it seems rather dubious that making API calls to a cloud server is an endeavour that takes hundreds of megabytes of code and resources.

If, rather more accurately, the point of a Google Photos app is to provide a photo editor and various other photo-adjacent functionality coincidentally including the ability to back up photos to Google's servers, then again that raises the question of why Apple's equivalent app is so much smaller. Are there image-related system frameworks that Google cannot use that Apple is using? Then sure, feel free to count them in Apple Photos' "true" size. But if Google simply won't use them then IMO it's fair to ask if the size of what they're shipping is worth it.