Older 125s and 270s often have the motor on the RH side of the chassis with a fuel tank on the left, though the modern ones have moved to a LH motor like the 600s. All modern (post 2000) 600's have a downtube cage in the front, whereas 125/270s are often just old-style cockpit only cages. Tubing diameters can vary, and often the smaller cars are narrower compared to a 600. I don't think any of this is by rule, just that as the 600's evolved they got bigger.