The Midland clearly had some peculiarities, superb (and heavy) carriages but until the Belpairs in 1902 and then the Compounds, nothing to pull them above a Class 2 rating. There was also the obsession with variations in wheel size that went against standardisation too. It seems they were parsimonious in their relationship with the Superheater Company, going the way of fitting to rebuilds where the royalties were cheaper. Yet they also went through the H Boiler "rebuilds" (as much a rebuild as the Patriot was of a Claughton) of their little 4-4-0s most of which were rebuilt with saturated belpairs long before the H boiler was life-expired. This clearly wasn't an exercise overseen by accountants, neither was the ornate livery constrained by costs. Johnson and Deeley both had plans for bigger engines before the 3P Belpairs appeared, unfortunately we don't know what the objections were.
It seems to me the small engine policy was management driven, most writers seem to blame Anderson, and in those days deference to seniority made getting round the objections and beliefs of senior people very difficult, even with evidence to back you up. I recall reading about the ready addition of pilot engines if the train became "overloaded", which indicates that the operations department were perhaps quite rigid in their thinking. A light Class 2 was a useful engine size for much of the network, both Midland and LMS, but not to go above a Class 4 passenger was not going to be sustainable to manage traffic even if the Midland had stayed independent.