A smaller diameter is weaker both in bending and torsion, no way around that. And it's not a direct proportion but something like the ratio cubed.

One can use a stronger material but because steel has a relatively constant spring rate (modulus of elasticity) that only means it won't break as easily, it'll still deflect the same amount regardless of the alloy.

If crankshaft strength were an issue it'd be worth considering. However, the crank, even with smaller journals, seems to be stronger than it needs to be.

R.