What I mean by this is that most traditional style Martial Arts will teach you techniques that have no real world application. Shit like defending against multiple attackers and fending off knife attacks without getting maimed yourself just does not happen. I have seen middle aged overweight women who genuinely believe they could fend off a 22st attacker using some bullshit wristlock that they've only ever tried on a compliant opponent in their tong po wang fu class.

Whilst I wouldn't say BJJ will help you in these situations either it does give you an amazing understanding of body mechanics and how to apply leverage to overcome larger opponents, it will improve your balance and movement which can be essential in certain situations. I personally do it for sport than self defense but knowing something simple like a rear naked choke has got me out of a couple sticky situations when I've been out in the past. The law don't look too favorably on you if you knock seven shades of shit out of someone but if you incapacitate them with a choke and make them go to sleep they don't seem to mind in my experience.

Don't worry about not being a big dude either. Jiujitsu was founded by a 60kg Brazilian dude called Helio Gracie. He was so confident in his technique overcoming his opponents physical advantages he would challenge all comers of all sizes and styles in Gracie challenges. You can see the footage on YouTube today.

I am slightly biased as I love jiujitsu and think everyone should do it but at least try it man and then make your own mind up from there.