"XXL category" does not exist, it a back yarder breeder term used to sell dogs. The American Bully comes in 4 classes, Pocket Class, Classic Class, Standard Class, and XL Class. The only class that has a difference in build is the Classic Class, all of the others maintain the breed standard with the difference only being in height.
At 8 months your PUPPY, is still not done growing yet. Bull breeds can take up to 3 years to complete growing and filling out.
High rears are not "normal" in any bull breed. If your grown dog has a high rear, then the breeder is not breeding to standard, they sold you a pet quality because they saw it would be an issue in the future.
ABKC American Bully breed standard in regards to back:
BACK
• Fairly short to medium back, slight sloping from withers to rump or straight accepted with gentle short slope at rump to base of tail. The American Bully should give the appearance of a square body, equal in length when measured from point of shoulder to point of buttocks and from the withers to the ground.
Faults: Back too long; rear higher than withers; weak or swayed topline; roached or wheel back.