There are big guys out there (a few that are really big) in the 121 world, but not too many. I don't know what the cause/effect relationship is, but here's something to consider: flying is great, but the airlines make it into WORK.

This means that you will, when the bloom is off the rose, want to be compensated for the work done. The only time you are going to make any money is down the line--when you are several (many?) years into your career. So to make the whole thing work as a career, you have to have a middle age for yourself that is healthy and productive. So even if you can get that 1st Class now, you MUST be prepared to get it again and again.

Bottom line-- you can and should lose weight, or the job won't work long term, most likely. As far as getting your foot in the door, the medical and a great attitude are all that matters. If you are a fun person and sharp, you're in.