Not everyone seems to be as understanding as this forum. I'm glad to see others will discuss the technical implications.
1. Double resources
2. Art/Aesthetics
3. Hitboxes
4. Gameplay (not rpg)
You would be surprised how many others believe these are lies to actually hide a social commentary. I can't tell the ignorant ranting vs trolling, but this this subject is often brought up regardless. I don't understand how anyone can insist it's all lies or how the decision must have social implications. They're not trying to promote masculinity, homosixism, or anti-feminism. They're just making a video game. Most of the reasons I've read seem to come from people not aware of their own social issues. While they want to believe they're a more pleasing character to look at, they also want more attention to the fact they chose female. The internet is never taken seriously so masquerading males are more common than actual females. Then others want to show how they have a girl friend as a female avatar to accompany their own male avatar. It gives them a social "one-up" to the other gamers who can't see that the relationship is being treated as an online trophy. Complaining how this game won't be that outlet for them is petty.
All Points Bulletin has the same hitbox for every character, regardless of height and weight. However, that does lead to odd situations quite regularly (e.g. shooting someone "through a wall", meaning "hitting his hitbox, but not his avatar").
That's the key to hit boxes that most I think do not understand. Ideally everyone should have a balance of surface area vs hitpoints. So the light characters are faster, thinner, harder to hit, but have less HP as a balance.
No game has been done like Brink before, so females avatars would have to be more than just reskinned males. Brink has enough factors where simply doing 1:1 for balance and social neutrality would not feel like Brink anymore. Most of the females would be light so that would be turning a size class into a gender. Meaning every time you spot a female they have the same clothing options, weapon selection, and agility. With experience you could predict the play style of any female if it were that way. While it may be closer to realism, it does have social stigmatisms. Heavy females being 1:1 with the heavy males would imply obesity and/or steroid abuse. Obesity honestly would not only have odd aesthetics, but also would not fit the game's fast paced guerrilla combat. Fantasy games usually get around this by making big strong females some orc or other "tough" race.
The only way I see heavy females is doing what's always been done in games; exaggerate everything giving appeal to carnality. All the females would look like characters from Heavy Metal. Since Brink seems post apocalyptic it would fit into place with everything else. But generally that sort of portrayal of females is frowned upon. It's clichéd now to do that in video games. How many would say "if I had known they were going to make them like that they should have not done it"? Both genders could be fairly represented this way. And if they do it this is probably how it will be done. So I can't imagine how "cute" or "petite" could ever emanate. I can't say what opinion women would have on that.
The strongest point I see is that females in Brink would be for more graphics/art and not contribute to the gameplay. Brink is not a RPG so no one is really missing out. Everyone could have been a gender neutral robot/clone/alien and the gameplay would not change.
I have high hopes for this game. Personally I'd buy a DLC release, but don't demand or expect one.
tl;dr
Females...
...not being present bother many
...make us wonder about hit boxes and other balancing
...should be in a DLC
...will be like Heavy Metal