Depending on what iPhone or iPad you are using, we cap the resolution to the native camera megapixel count in the phone 6s, 6s+, 4, 4s, 5, and iPod Touch and the same with the iPads. The exception is with the 5s, 6, and 6+ that shoots 8mp. Mextures can edit/export images on these devices at 11 mp.
The iPhone 8, X, XR, XS, XS Max, 11, 12 (all models) (13 all models) and 14 (all models) can export at the native resolution of the photo and is not limited to the megapixel of the device!
As Apple increases the memory allowed to each app when it's open, and camera resolutions change, we will keep updating and fine tuning to enable higher resolutions to be edited on older devices.