Blob,
your two locomotives are actually a fine match..and they are roughly the same scale..
IMO, dont even worry about it!

they will go together fine..
your models are different sizes, because in real life locomotives came in many different sizes..
even locomotives on the same railroad, at the same time..
Here is Kevins excellent comparison photo:

Those three locomotives are the same scale!

If they were real locomotives, and not models, they would have the same proportion to each other
as the models do..happens all the time..locomotives are a very varied bunch..
So, IMO its all really a non-issue in this case..and IMO, there is no quandary at all..
Scot