The market is flooded. Flat out, there are probably 40-60 people qualified for every job posted. In NYC there perhaps 1,000 stage managers competing for the 47 or add jobs in commercial theatre. It's a fact of life. Too many schools are putting out too many stage managers, many of them not prepared for the hard reality of this job market.
And regarding bigger size and acting - well, theatre quite frankly is a visual medium - and your size says something about you when you are put on stage. Can you imagine a production of Romeo and Juliet, where the Romeo weighed 400 pounds, and everyone else was normal weight? It would say something, you can just slip it in. The rough part about acting is it is based on looks first, talent second . . . hundreds of headshots may be soured before scores of auditions are scheduled. Like it or not, that's the medium.
Show Business is an EXTREMELY hard business to make it in. It requires a lot of hard work, patience, drive, commitment, education and schmoooooozing. It's not a fair business at all - NOT AT ALL. 99% of the jobs are never posted, never advertised never take cold submissions.