This is is using a scale factor of 1.0 and, to be honest, it could even do with being ever-so slightly bigger:
If I freeze the game and fly up to the cab, the player looks a little big to fit inside comfortably, but not too bad.
Remember it should accommodate two in reasonable comfort:
Spoiler:With a bit of tweaking to up the scale a bit, that could easily be resolved. The problem really comes (IMO) when the AT-ST starts to move. I've set up a simple walking cycle which looks good, except for the rapid change in direction through 45 degrees that is the standard Doom chase pattern. It's acceptable in man-sized enemies (IMO) but as the actor gets bigger and bigger, it looks increasingly odd. Having this towering mechanical enemy snapping between 0, 45, 90, 135, 180 etc degrees as it moves looks odd IMO. (Of course, it's even worse in long, low enemies - like trucks or something.)
There is also the issue that, visually, things should be able to pass between the legs of the AT-ST but, if you are going with standard Doom collision, the enemy will be a giant cuboid block. (Cunning collision stuff may be possible with advanced scripting/collision detection - but that's beyond me).
In short [ahem] the engine isn't really set up to have moving actors of this kind of size.