by big weights, i mean powerlifter routines - bench press of +120kg, deadlifts of 200kg, that sort of thing. perhaps it was a little unclear. an yes it really does have an effect on flexibility, i know several weightlifting coaches who say that they regret losing their flexibility - the muscles can become so big that it inhibits movement. tho most people don't intend to ever get to this 'big' stage, some of them end up there after getting carried away.
you get added strength from flexibility training if the stretches require you to support body weight. there are a few that we do for kung fu that you probably won't have seen - not gonna go into detail because it would take a long time, and i'm not sure what i write won't be misinterpreted. i would agree that most of the stretches done sitting or lying down won't have much effect.
cardio work done outside if its wet and cold can be bad, if you breathe in and out through the mouth, because cold air goes to the lungs, without being properly warmed up (as it would be through the nose). thats why tai chi should never be done outside when its dark or rainy. maybe there is less effect when jogging or cycling etc, but my teacher always said not to. if that doesn't satisfy you, best ask a doctor of chinese medicine, they might be able to explain it properly.