When introduced in Sword & Fist, the Maul was a 2 handed martial weapon.
In CW, its a 1 handed exotic weapon (usable in 2 hands as a martial weapon).
In both sources, it weighs the same 10lbs.
The Warmace, introduced in CW, is 1d12 and 20lbs.
If you must, you can HR a switch between stats of the 2 weapons, so the Maul becomes a 20lb 1 handed exotic weapon (usable in 2 hands as a martial weapon) doing 1d12 damage and taking the -1 to AC while the Warmace is the 10 lb 1 handed exotic weapon (usable in 2 hands as a martial weapon) doing 1d10 and having no AC penalty.
This one had bothered me for quite a while, until I thought about it thus:
A typical sledgehammer for construction/wrecking purposes has a 8-10lb head. I bowl with a 16lb ball. The difference is that the 10lbs is at the end of a 2.5-3' haft, while the bowling ball is in my hand- physically similar to the Warmace in that the mass is located close to the hand.
And yet that sledgehammer is a little easier to swing around than that bowling ball. Its made to be swung around at a target at 2x arm's length.
If anybody wants to try an empirical test, go to your bowling alley and ask if you can have the most beat-up 16 lb (maximum weight allowed by the rules, btw) house ball they have for an experiment. Then just find a way to put a heavy-duty handle onto it. Or, of course, you could try swinging a 20lb barbell around in the gym (if they don't kick you out...). It will probably feel VERY unwieldy.
I suspect that unwieldiness would dissapear with training, however.
However, in D&D, there's no difference in a weapon that extends your reach 12" and one that extends your reach 3'..."Reach" doesn't exist as a weapon quality until you're talking 5' or more in length.
So...in order to make the 2 weapons balance out, it was neccessary to penalize the Warmace, since the Maul didn't have any additional benefit over the Warmace, despite its lighter weight and longer reach.