Don't worry. For example, check http://www.thinnertimesforum.com/topic/54606-q-to-those-that-dont-need-the-seatbelt-extension-in-airplanes/

> I did not need to wear an extension when I flew last time - I am 5'9" and weighed 260ish pounds. I was in an 18-20 size clothes.

> last time I flew, I was prob around 300 lbs. (or a bit higher) and I didn't use an extender. Never have actually, and my highest was 330lbs. Size 22 or 24 pants at lane bryant and I'm 5"6.