I find there is a divide between bears and twinks, speaking from personal experience I can tell you that these days if you're not beefed up and muscled, nobody looks twice at you. I personally feel that within he last few years, gay men have gradually been more exposed to media and advertisement where it's beefy, beary, masculine looking men promoting club nights as opposed to other models with different body shapes, they're also way more common to find in porn (although to be fair, there's a porno out there for everyone, you could easily find a porno featuring twinks if you looked for one).
I've found with a lot of "bear" types I've met these days there's an element of superiority about them due to the fact they look attractive and masculine. Being young, with a slim build, I have often noticed that some bears don't want to be seen near me in clubs because (and this is only the impression I get) they feel like I'm not worth their time if I don't look like them or behave "straight acting" (which I personally think is desperate trying to impersonate another orientation). I know a lot of naturally skinny guys who really struggle trying to gain weight and they've been putting themselves through insane diets and gym sessions trying to beef up so they can fit in.
There is an unspoken rivalry between the two. Some bears are cool but I know a lot of them who think acting camp or looking anything other than masculine is disgusting and I know just as many twinks who are increasingly becoming more body conscious surrounded by all these buffed up men.