My mothers sheltie is the same way, he's about 35-40 pounds. I put him on a diet along with my beagle Hannah and Hannah lost 15-20 pounds and Tucker lost nothing. The vet said he was fine put him on low fat food and give him plenty of excersize. That was about 4-5 months ago and he hasn't lost a pound. He is happy and active but he is a big boy, my dads sheltie is so tiny and they have always eaten the same food and the same amount and gotten the same amout of excersize ??? We can't figure out why he's so big.