Well I guess they just used lighter metals and seeing as there wasn't to much un-balancing.You would find that when the piston is at TDC the weight on the crank is it it's furthest away point therefore keeping it more balanced,so there really wasn't too much unbalanced weight being thrown around.Then relied on the cutting disc,to take up what was left.My tilt-a-cut with it's power-torque engine has as balance weight under it to keep it in check.
I'm not a real fan of the power-torque but who ever deigned it did seem to do their homework on it.
