Riley is about 80 pounds at about a week short of 7 months right now. He was 68 or so at 6 months, I think (?). I'd have to look. That's under the "average" on the only chart I've found on the internet (the one someone mentioned above). But he's been slightly behind that chart all along, and it's gotten to be a slightly bigger difference. However, he was growing at about 3.8 pounds a week from mid-October through the end of December (3.5 months to 6 months old), which is very fast. I'd rather he didn't grow that fast, as it's easier on his joints.
I figure, he'll get there eventually. If he grows slower, great. I wouldn't be concerned about 57 pounds at 5 months... That sounds pretty close to what Riley was, though I'd have to look, like I said. His mom was 135 and his dad was 185. But, hey, he could stop growing today and it wouldn't matter to me... well, aside from the panic of "Is something wrong? He's not even 7 months old..." LOL.