Well pollinated and well fed cherimoyas can attain that size,
but pollination is usually deficient, often producing smaller fruits than the ones shown in your picture.
So, I'd say regular size is smaller than that mammoth, but I've seem many examples of cherimoyas of that size.

MMMMMMMMMMM yum yum gonzer , one of the first fruits I tried after shifting to the sub-tropics . Had a good sized tree in the back yard of the house I lived in , and it had an enormous crop . Maybe not Cherimoya but a good custard apple. It never got as much fruit in the 10 years I lived there. Enjoy your bountiful harvest.