Ah....it's for the big stuff only then!

So....when I see threads where folks are discussing what engine to put in, and one guys says "i'm putting in a Saito 1.80" and is followed up by the next guy "i'm going to put in a g23 or g26 instead", I have to wonder why? Is it like the difference between a 2 stroke and 4 stroke glow?
Or when you get to a certain size it becomes a personal choice? Or do aircraft models specify gas or glow at some point?
There is one fellow at the field I fly at that has a gas engine, it sounds like a chainsaw....seems to have lots of power though.
What about multi-cylinder engines...i've seen them online offering as many as 9 cylinders to a single engine!!!!!