I'm just going to be painfully clear on a point about weapons here, there is an absolute maximum speed at which you can possibly swing any weapon, ergo, to increase the destructive power of said weapon past that point, it must become heavier.
Yes, but simply concluding that more mass = better only works when you're in a reality where you don't have to account for exhaustion in battle or from marching. A game character can carry around a 20lb mallet indefinitely as long as it's under their weight limit and swing it for as long as the game allows (in the case of Skyrim, forever, or with power attacks until the stamina bar drains, at which point you need to wait a few seconds). In real life marching around with a 20lb lump of metal strapped to your back -- ignoring the spinal problems that would cause -- would be completely impractical for a soldier who also has to march around in armor, carrying secondary weapons and personal supplies, &c. Likewise, using a weapon like that in a real battle would both be a death wish (when you swing a 20lb lump of metal attacked to a stick, you leave yourself
badly open), and you wear yourself out incredibly quickly.
You're also wrong about mass being the only way to increase the kinetic energy behind a strike, and the answer is patently evident if you take five minutes to do the research.
Leverage. You know what two-handed warhammers looked like in real life?
Exactly like one-handed warhammers, except with a longer shaft. If you've ever done any sort of construction work, you know how effective increased leverage is as a force-multiplier. It's used as a colloquial expression for a reason.