I flew in November (I was anywhere between 270ish - maybe pushing 280). I have large hips. The seats were very uncomfortable. I was definitely squeezing into them. I didn't need a seatbelt extender, even with my 3 months post partum belly. This was on Continental. I'll be flying Contintental next week and wonder how the seats will feel this time. My flight out was entirely full and I was holding my 3 month old, no one was flying with me. In addition to trying to stay in my seat and not slop into the seat next to me, I was also trying to nurse my daughter descreetly, in a middle seat, not fun! I had great passengers around me though (the guy originally slated to sit next to me did trade seats though). On the way back, there was one empty seat and it ended up being next to me. I was thrilled.

Arm rest went up!
I have flown at 13 weeks pregnant and about 285, didn't need seat belt extender - flew Southwest.
I have also flown at 8 and 9 weeks pregnant (around 275) on Delta and Northwest. Uncomfortable, but DH was next to me so we snuggle.

We got bumped to first class and those seats are NICE - unfortunately, the plane had engine trouble and we never took off and we got sent back to Coach on another airline.
Flying smaller is definitely easier, but although uncomfy, I've not had to ask for a seatbelt extender OR pay more.