during a trip to mexico last winter a friend of mine had horrendous problems with her brand new rope kinking up and knotting on her. she did everything right to the rope for the initial uncoiling of the rope - i suppose sometimes you just get a bum rope??? that sucks 'cause you can't return them usually.
anyways, i find that smaller ropes do tend to kink up a little bit more whether it's because of design, or handling i couldn't say. but i find that it's not a huge problem if you pay attention to how your ropes are running through rap anchors, tr anchors, directionals and belay devices...if you keep the rope in a single plane during use then twisting rarely occurs. i find that when people are rappelling with a tube style belay device and they have it rigged through the waist and leg loops of a harness (belay slot orientation side to side) as opposed to using the belay/rap loop that's standard on 90% of all harnesses now (belay slot orientation up and down) there seems to be more kinking problems.
in my opinion a lot of it has to do with bealy/rappell technique. if you belay with the device in an "up and down" orientation but you hold your brake/feed hand out to the side, you end up putting a 90 turn in the rope's path and that kinks it up. likewise if you rap with the device in a "side to side" orientation and then you hang the ropes straight up and down between your legs you're putting a 90 degree twist in the ropes.
also if you tie the loose ends of the rope together during a rappell they twist like all hell. i usually tie back up knots in each individual rope before i toss them. and if i know i'm going stright to the ground i don't tie knots and just try to rap all the way off the end of the ropes (i.e. feeding them through when i'm safely on the ground). also try to make sure that you're loading the ropes while feeding them all the way through teh belay device as opposed ot just unkinking them and feeding them through. the "load" on the anchor side of the belay plate helps to straighten the ropes as you pull them through.
whew...ok i'll stop now.