why does my iphone storage keep fluctuating?

when i first got my phone (iphone 15 pro) and transferred all my photos and stuff, my storage seemed to fill up super fast. even though i had more storage on this phone compared to my last (like 100+ gb more), my new phone couldn't even transfer all the photos i had. so i deleted thousands of photos (9k+), but my storage just didn't seem to move at all. then, suddenly, it said i was only using 45gb of storage. so i was like, "what the heck? why did i delete all those photos then?" now, i have around 6k photos (which is way less than the 15k i had on my iphone 13). but today i saw that my phone is now running out of storage?? again?? i swear i checked yesterday and it, again, said that i was only using 45gb (meaning i had around 200+ gb of free storage space). i don't understand why it went up when i haven't even taken that many photos either. photos are the only thing on my phone that take up the most space. i also have more storage on this phone, so why is it that my iphone 13 can hold 15k photos, but my iphone 15 pro can't?