I think the thing you're missing is firebox depth/volume. The firebox has to have a minimum volume. On a locomotive with a narrow firebox, the bottom of the firebox sits on top of the frames between the drivers, making it relatively deep and giving it adequate volume for the grate area. On a locomotive with a wide firebox, the bottom of the firebox has to be above the tops of the drivers so the firebox can be wider than the track gauge, or it has to be located behind the drivers over the small wheels of a trailing truck. The whole boiler ends up sitting fairly high in order to give the firebox adequate depth and volume.
Another way of getting around this was to provide an immense combustion chamber in the boiler (an extension of the firebox into the boiler barrel). Look at a Challenger or Big Boy and you'll see this. The bottom of the firebox is just about even with the bottom of the boiler barrel. They provided adequate combustion volume by providing a huge combustion chamber.
You don't slap a fatter boiler or bigger drivers on a locomotive just because it doesn't look right. The buyer said "we need a locomotive capable of pulling XX cars on an XX% grade burning xxxx coal. The maximum axle load is XX,000 pounds, and the maximum engine weight is XXX,0000 pounds. It should be capable of negotiating a curve of XX degrees and capable of at least ~45 MPH" (or whatever). There'd probably actually be a lot more criteria than this.
The engineers at Baldwin, Lima, or ALCO then used their experience and engineering knowledge to juggle proportions to come up with a locomotive meeting those criteria, and you can rest assured that #90 met its design requirements very well.
Hugh "Low Water" Odom
The Ultimate Steam Page
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