Rottweiler experts please help..my 1yr old Rottie's weight gain?

I have a 1 yr old female spayed rottie. She now weighs in at 55lbs. If you go by the rottie growth chart, she should be 70-80lbs. She was the runt of the litter mind you. I'm still concerned, she eats 4-5 cups a day of premium Canidae kibble, she's healthy by vet's standards, so I'm not sure why she has the height, but hasn't yet widened out into that rottie type body. I ran into an 8month old rottie at the dog park yesterday whom i've know as a pup and he's like 80lbs already, and thick.....why is my baby-girl looking more like a doberman than a rottie? Will she thicken out over the next couple of years? I confirmed that she is a pure rottie with the breeder, saw the parents too, they were huge...what gives? any suggestions? I mean she's like 40lbs off the mark, even a runt should only be maybe 20lbs off at most...is it possible for a runt to be that far behind? I had her spayed before her first cycle, but even at that, it would keep her this tiny...she's