You're probably using a "european" tape measure.. hehe. The only think I can possibly think of is you're putting on muslce mass and getting bigger that way. You haven't mentioned anything about working out, so that's only an assumption. I can't see how anyone could add size and not weight if it was FAT related (w/exception of a few lbs in water weight). Most people get larger from lack of exercise as your caloric intake exceeds your daily expenditure.

There's different ways to check weight and fat. 1) scale. 2) tape. 3) clothing fit. 4) skin fold caliper. 5) impedence tool - electronic gadget that tells body fat % by electrical impedence in skin. and lastly 6) water submersion testing - expensive and a pain, but very,very accurate.

Good luck!