Thanks for your answers.
Of course, the small one should produce less fuel per a unit of time, than the big one. No questions. But overheating and wasting ore (despite limitless resources on planets)? I assert that they take place.
Guys, KSP is a simulation game. It's a question of being authentic.
I just want to know WHY Convert 125's made in the way we see it now. Cpt Kerbalkrunch, sorry, what do you mean by "post directly"?