If you gain muscle mass you will still get bigger...same as fat gain. What is your diet like? What is your training like? If you're consuming mostly lean protein sources like chicken, turkey, steak, lean ground beef, tuna, etc I wouldn't worry about it.

Oh ya, and how much weight was it and how fast? I am gaining about 2lbs per week while on my weightgainers with a small percentage of that being fat. Other people have gained even faster then that.