The W1 cab is only larger in length, not width, in that aspect it is identical to the A4
There's a couple of reasons, so treading the thin line here I'll try to explain, most of the issues are to do with modelling so whilst it might appear 'wrong' it's just a combination of small things to make life easier when modeling.
The issue is the width of the cab, it should be 98" inside sheets, or in our terms 57 mm as near as matters. Unfortunately the resin casing where it meets the cab is a tad too narrow, shrinkage, whatever, it's lost in the mists of time. When the W1 casing was worked up this factor was considered but the work required to widen the firebox area was significant, it's not just the firebox, its the combustion chamber and boiler rear tube plate area as well.
To put this into context, we're talking about a 0.8 mm overall gain in width, it's that small, mind that would be large for some, however no one had even noticed on all the A4 kits sold, not that I'm aware of.
Taking the A4, the cab is the correct width as far as I can remember, it might even be a little wider, should be 57 mm between side sheets so the upshot of this is a slightly larger step between the cab front and firebox cladding.
When I worked up the W1 this step was simply not appealing, here's the real deal.
On the real engine it's about 3" and the A4 is pretty much the same.
So being as the casing is an unmovable object then the cab sides had to be altered, in our case I trimmed 0.3 mm off each side, it should of been more but this was the best of both worlds.
This now means the cab is 56.4 mm between side sheets, I suspect no one would ever have know if I hadn't of said so.
The next issue was the cab windows, one of my pet hates is glazing and how difficult it is to try and 'stick' it in after painting, so a different approach was taken, one where you can paint at will and then simply slide the glazing in, no gluing, sticking, faffing around, just drop it in.
To do this requires the sliding windows to have an internal recess, the real engine has two sliding windows so to keep the thickness down, one side of the sliding window was fabricated from 0. 4 etch with a 0.2 mm recess and a half etch 0.2 overlay.
The overlay isn't shown but the 0.2 mm recess is in the full thickness part, the window runners even had two individual tracks for the frames to slide in, it all worked until you painted it.
Back to square one, bring as the front window is rarely opened it was forsaken for a fixed window, this drops down between the cab sides and overlay and being as the cab sides are 0.4 mm thick then all should be well. The sliding window was then fabricated from two full thickness laminates which now leaves a nice 0.4 mm gap for the glazing and the reduction from two 0.6 mm windows to one 0.8 leaves a little more room for painting and still having working windows.
All of this means that the window frames put on a little weight, about 0.5 mm overall. Incidentally the front spectacle frames are still 0.2 mm gaped so care needed there when glazing.
The glazing slots can just be seen in this close up, spectacle and front 'fixed' side windows, the sliding windows are not fitted. You can also see where CCU stuck his oar in with the backhead extension plate, I've adjusted that on the master etch to close the gap, but as noted previously, you'd never see it once the roof was on.
So the upshot of all this is that I now have a 54 mm backhead trying to squeeze past a 53.4 mm opening between the window frames.
The end result however is, much better visuals twixt cab and casing, much easier glazing facilities and I think that kind of offsets the modelling compromises, others mileage may differ.
Regarding cleanliness, all of this is hidden soldering or surfaces that are easily cleaned afterward.