Hey there, and welcome to Apple Support Communities Ssoley,
From our understanding, pictures taken on your iPhone 13 Pro Max comes out pixelated on your iPhone 12 Pro. We can certainly understand your concern and are here to help.
We would like to ask you a couple of questions in order to get a better understanding. First, what exact version of iOS are you currently using with the iPhone 12 Pro? We ask as the menus and options can change slightly between versions of iOS, and we want to make sure we provide the most accurate steps. You can locate this by going to Settings > General > About. Also, does this issue happen to other devices you send your photos to?
We thank you for your answers, they will help guide us to the next troubleshooting step.
Kindest regards!