I think it's actually a question of contact area. See, hammers are really good against armour, but not nearly as good (relatively speaking) against bare flesh. This is because a small contact area with a lot of weight behind it focuses a ton of force onto a single point, which is important if you're dealing with something really hard. The tradeoff, though, is that you're less likely to hit a vital area. The bigger your contact area, the more chances you have to hit something interesting. It's better to hit something unimportant and actually damage it than to hit something vital and have your attack glance away... but if you're strong and skilled enough to do full damage with every hit, large surface areas are best.
If that's true then mauls should be pretty good against unarmed opponents as well. It's still a lot less than a crossbow (100 instead of 10,000) but the huge size might make up for it.
Also, how does velocity comes into this equation?
From what I understand, velocity just multiplies the weapon's effective size. An axe and a sword might weigh exactly the same, but the axe will hit harder because most of its weight is concentrated in one end.
Size makes a weapon strike with more force; it won't change how likely you are to hit with it. A maul (size 1300 (x2), area 100) will cover just as much area as the pommel of a shortsword (size 300, area 100), but it'll hit much, much harder.
The damage equation, as I understand it, looks something like
Size * Velocity / Contact Area. Contact Area and Penetration determine the width and depth of the wound, giving a blow more chances to land squarely.
So yes, a maul's larger surface area should make it more effective than a hammer if armour isn't a factor. That said, 100 is way less than (e.g.) a battleaxe's 40000 area. The maul's size will still count for something, but edged weapons have the extra benefit of being able to cause bleeding and sever limbs. Against unarmoured humanoids, a greataxe or halberd will blow the maul right out of the water.
(Oddly, if we plug the hammer and maul's stats into my equation the hammer actually comes out on top, damage-wise. The smaller surface area really counts for a lot. Of course, there's a good chance that my calculations are at least somewhat off: maybe it actually uses the square root of the surface area, or maybe there are some hidden constants somewhere. I haven't had a chance to do much testing.)