Having seen the East Anglia Stadler fleet up close and personal, I think I know why they look "European" vs other "British" trains.
Apart from the more restrictive guage and the general "pinching in" at the top and bottom of most British trains, one characteristic I foudn common on british trains, vs Continental Trains is the fact that the "body" panelling of the british train usually starts at the "floor" of the train. Maybe this is due to tradition (body's were built upon a flat base that is at floor level and above the wheel tops) and practicailty as the guage is usually quite restrictive bellow the floor in the UK. The vehicle below the floor level is also often uncovered and a different colour to the body (i know some High speed carraiges/trains such as Mark 3, mark 4, etc have skirts but they are still of a different colour) This gives a clear demarcation of where the floor is and is common on most British trains. Even the D-Trains (and top some extent the the A Stock) look very british, despite the flat sides, as the sides stop at the floor level.
This can make the train look a lot smaller than the restrictive guage would suggest. This also applies ot the front cab where especially the older Classic desiro (both gangwayed and non gangwayed) look a lot more "squat" than an electrostar (whilst appearing wider as they dont pinch in as much on the sides)
The stadlers on the other hand have a body "panel" than goes below the floor, is more european, despite it being designed for the UK Guage. In fact you dont immediately notice how much smaller they are until you see then side by side with a european counterpart.
As an interesting side note, some American carraiges also have the side panels starting at floor level, and sometimes look a lot smaller than they really are.