Height**
That pic does give that appearance but when there's others next to each other where you can tell Nick has some more thickness to him.
And the exterior muscle isn't what's the most different, facts are Nick is much more dense than Nate. Nick fought at "LW" very early in his career but mostly fought at 170, where he has stated how draining it was and a 175-180 catchweight is his ideal weight.
Where Nate was an undersized WW who has never seemed to struggle with 155. He says he likes that's weight and feels good at it. The RDA fight was clearly personal issues, considering he never missed before.
Nick is legit one full weight class bigger. Its not "huge" but, Nick wasn't at a size disadvantage against Silva, Nate would have been. Idk how else to put it lol