I have always heard to double the weight at 4 months for an approximate adult weight. I have found that to be true to get their approximate 12 month weight but for boys, (that is mostly what I have had and remember!), they continue to fill out and build muscle and bone mass for up to 2+ years. It has meant my boys have been heavier by their 3rd birthday. Maxx is definitely a 90+ lb boy of solid muscle. He also has a very heavy coat which makes him look even bigger, many guess by 10 lbs! Often people are shocked when they put their hands on him and find he is not fat!
And absolutely, they can look like their grandparents. Maxx does look almost exactly like his dad, big old head and all. Genetics are funny, you never know where a trait will show up!