Welcome to YorkieTalk
First, I am not a breeder, so I"m no expert.
It depends more on the body struture then on weight. Did the female come from a line of free whelpers?
Before your parents breed, are they planning on doing all the testing that should be done (LP, LS, STDs, hips and shoulders checked, eyes and ears checked...). Yorkies are a lot of work to breed and I'm sure they don't want to be breeding dogs with "issues". Please also make sure they check their bloodlines for LP and LS...