I'm going to be honest that I haven't actually looked into this, I'm just making an informed guesstimate, but to get Maxx battery life, you'd probably need a battery the size of the Maxx, right? Well, haven't the batteries for the Maxx been oh so slightly bigger physically than the normal RAZR? (I just remember it being pointed out for the originals, and after seeing them in person, it was noticeable; not sure if it's the same for the HD's) The DNA sandwiches the battery between the display and circuit board; now I don't know how many other devices do that, or if it's a new thing, but it is safe to assume that a physically larger battery could potentially cause some problems with that. Maybe a 2,500 could have worked at the same size, but then there's the deal that this phone is $200. That is a fantastic price, and cuts were very likely to be made to keep it at that price point. HTC probably was along the lines of, "Let's try to get as small a battery in, that provides comparable/decent/average/good enough (for them) life."
I'm also going to be completely honest that I know pretty close to nothing about rooting, but wouldn't getting rid of Sense increase the battery a fair bit anyway? Not to mention the serious improvements a ROM could make to battery life.
I'm not sure what you mean by "appropriately sized," but I'm sure with everything else in the phone, if it was 3,300, at the very least, it would not be $200; if it was 2,500, it wouldn't get HD Maxx results by itself anyway.