my new baby girl seems too skinny . she is 10 weeks and eats only 2 cups and a small can of food per day . her two brothers are huge copmared to her. she weighs 12 lbs (they are 4 lbs heavier)is it because she is a girl ? i only have had boys . what can i give her to fatten her up . the vet does not seem to think there is any problem ,it is just me . am i worrying too much?