i think it dpends on what type of exercise you are doing. When i first started heavy weightlifting a year ago (HEAVY- 100 TO 160 POUNDS) my pants became tighter. And then i started gaining weight (but that had nothing to do with the lifting, i was eating too much high carb, high cal food). Since starting atkins last december, and by getting harcore with my HIIT cardio 5 days a week, i have lost a lot of weight, and the muscle i have been building is really starting to be prominent.

We do not gain a HUGE amount of muscle in a short time (not enough to add a pant size...) So if you're still gaining in a couple of weeks, i think you should look towards your diet.