I'm pretty sure it also has something to do with the pose, lighting, or coloration of the figure. I'm sure they're just reusing the same sculpt for each of the Gokaigers instead of altering the heights at all, so it shouldn't be any skinnier than GokaiRed.It's because he looks as tall as SHF Gokai Red, while in the show Gokai Silver is shorter than Red and Blue.
I apologize if this has been posted elsewhere on the board, but I thought this is relevant to this thread:
Power Rangers 6 inch Shinkenger Action Figure - Gold
Power Rangers 6 inch Shinkenger Action Figure - Red
Edit: Someone posted it in the samurai toy thread already, sorry for the repeat then D: