Brothers tend to be of similar sizes, as do fathers and sons. It's not always the case though and we do have some examples of noticeable size differences. I'm mostly looking for people within the same generation or at most one generation apart. That a grandson is much larger than his grandfather or the opposite happens (especially when players were smaller 50+ years ago).
Tom Pyatt: 5'11", 185 lbs
Taylor Pyat: 6'4", 230 lbs
Saku Koivu: 5'10", 181 lbs
Mikko Koivu: 6'2", 213 lbs
Jordan Subban: 5'9", 181 lbs
Malcom Subban: 6'2, 200 lbs
Greg Malone: 6', 189 lbs
Ryan Malone: 6'4" 224 lbs
Nick Robertson: 5’9 178 lbs
Jason Robertson: 6’3" 202 lbs
These are the ones I could come up with. I'm sure there are others but I think it'd be hard to top out the size disparity between Taylor and his little brother. 5 inches and 45 pounds is pretty insane for a pair of siblings.