When he made his TV debut in Season 4 (along with the rest of the railway), his face was only a little thicker than it was in The Railway Series. However...
In the very next season, new versions of the engines were built in a much bigger scale, but they still ran on O gauge. The proportions of Rusty's face were messed up unlike the steam engines, which led to his whole front (or rather, most of it) being taken up. It was never corrected and got carried over to the CGI model as well. That version replaced the original for the rest of the filmed era, and the TOMY and TrackMaster versions were modeled after it. Oddly, his face briefly went back to its previous size during Season 12 until it grew back again:
Also:
Is that a TrackMaster hopper body on a TOMY tanker chassis with red wheels? Unless it's a custom, I've never seen that variant before.