Giants can't trample?

[Agema]

Trampling is a way of attacking by intentionally crushing opponents with the body, not just a factor of being big.

What probably matters more for a trampler is a certain disregard for pain and personal safety. As opponents tend to have sharp pointy weapons of their own, charging in and using sheer body mass to kill them probably involves a lot of pain via incidental cuts, and the chance of getting isolated, surrounded and cut down. So reasonably, although many giants could trample smaller creatures, they find the experience sufficiently unpleasant that they'd rather swat them with oversize weaponry instead.