Difference between thick vs thin front suspension?
Fork-leg diameter is not really how you would classify suspension forks. Or more exactly, it is a way, but not really the most relevant.

Suspension forks could be classified by how much travel they provide (different travel for different specific uses), the suspension mechanism (springs, fluid, elastomers, air, hybrid), the damping mechanism, etc.

I suspect that both those bikes were pretty cheap, both probably use spring suspension, and neither is really intended for hard off-road use. A bigger-diameter tube isn't necessarily heavier, as it can be made with thinner walls.