The two domestic shorthairs my husband had when I met him had pouches, to me it is a very domestic trait. My British Shorthair has a decent one but then he's chubby all over 
As Patti points out, it is a well-documented trait of Egyptian Maus...and so in some Savannahs they might inherit it.
Completely agreed that your girl should be spayed, not because of any fears of brittle leg bones, but to prevent the behaviors that most pet owners don't appreciate: yowling for a mate, spraying, desperate attempts to escape the house to find a mate...
As Patti points out, it is a well-documented trait of Egyptian Maus...and so in some Savannahs they might inherit it.
Completely agreed that your girl should be spayed, not because of any fears of brittle leg bones, but to prevent the behaviors that most pet owners don't appreciate: yowling for a mate, spraying, desperate attempts to escape the house to find a mate...