Yeah, American box art is never good to go by when it comes to Sonic character design comparisons. Same goes for the EU boxart, the Sonic 3 cover depiction of Sonic is fugly.
Regarding ezodagrom's links, the major change took place between Sonic 1 and 2. They reduced the size of Sonic's head so that he wasn't so top heavy. Forgot the ratio of head to body, but I think it went from 1:1.5 to 1:2. Sonic's design was pretty consistent on cover art from Sonic 2 through CD, 3 and S&K and even Sonic Jam. The only major different designs popped up in the Sonic CD animated scenes, which squished the eyes, simplified the design (ball hands, less lines) and made the sock and glove rolls more prominent:
Even then, the Sonic CD intro design was very much in line with the cover art at the time.
The biggest change in design came with the in game model of Sonic in 3&K. This in-game design was definitely the furthest from character design seen on the Japanese package art.
From then on, despite the standard Japanese design being used in subsequent games (Sonic Jam, Spinball JP cover art) the in-game design was all over the place.
When the games went 3D we had this guy representing Sonic in Jam, R, Saturn 3D Blast's special stages and very early stages of Sonic Adventure:
The point I assume I'm getting at is that the real deviations from the standard in-game design were Sonic 3 & Knuckles and Sonic Spinball, which were more in line with the US art. The pre-Dreamcast 3D games had very similar 3D models, which seemed to go for that Sonic 2/CD look. Beyond that it was a mix of in-game models that ranged from good (Sonic Adventure Dreamcast, Sonic Adventure 2), Bad (Sonic Adventure Gamecube, Sonic Heroes) and Ugly (Sonic '06).
It wasn't really until Unleashed that they found that, in my opinion, ideal mix of classic and modern. It seems SEGA likes it, as it has essentially been the go-to design for Sonic. Same goes for the box art model, as little has changed from Unleashed through to the latest renders released for the 20th anniversary. 4 years of the same design? That must be a franchise record!
