Beauteous, don’t you mean, ahem, substantial or something less complimentary?
You can choose. Here are the synonyms for big and large...
generously proportioned, outsized, overweight, fat, chubby, obese, corpulent, heavy, portly,
considerable, not inconsiderable, significant, generous, copious, sizable
I pass on the use of generous though; while her butt size may be, her spirit is definitely not.
The canine is very nice looking too.