Girth is the measurement all the way around the snake 5" is not all that big for a 2-3' ball. I've also noticed that the females will get fatter than the boys too, so that may be why your brothers ball seems a bit skinny. Also I think you said he was only eating rat pinkies every other week as compared regular weekly feedings that most of the other snakes are getting.
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Tosha 
3.7.0 Ball Python (Harry and Fluffy and currently un-named)
0.1.0 Siberian Husky (Kita)
0.2.0 Feline (Pippen and Pandora)
0.0.1 Dessert Tortoise (Pope)
5.4.2 Fish (1,2,3,4...)
0.0.1 Frog rescued from pool skimmer
0.0.2 Lizards rescued from pool skimmer