I don't think MP Raiden is stubby so much as thick (or thicc
). That's where the Alternators reminder comes in - they had to have near-complete interiors (without robot parts crammed in) which made them all pretty bulky in the torsos. Unlike the 3P Raiden that is able to cheat in any number of ways and sacrifice perfect-scale, Takara had to be able to not compromise all those small components that make it work like a train car. The bogeys alone add a bunch of kibble that is hard to hide, and therefore interferes with stashing other parts. Meanwhile, Radiatron just has little bogey-shaped tabs that fold neatly out of the way.
In the end, it's just a different set of priorities and there are still budgets to be met. It's funny that Tak has gone all in on cartoon accuracy in the rest of the MP line, but this team went the other direction for this piece. I recall the early articles mentioning just how big the train enthusiast community is in Japan, so they must have decided it was worth it. Ultimately, were I to buy one of these, I'd choose Radiatron, but I'm not a train guy; maybe I'd feel differently if I was.