I found this fascinating quote today:

People who have weight issues are starting to “segregate” themselves, report saysTIPS BOX, Classes of Yoga for Overweight People Are Becoming More Popular, May 2009

As a yoga teacher of size, I would say it depends on the individual, the type of class, and the teacher.  Not all yoga teachers are trained to adapt for the larger body, or for other physical challenges.  Some yoga teachers even discourage students from their classes because of their size.  But if you’re big, and have the self-esteem to handle a mixed class, and you have a good feeling about the teacher, (not just her expertise, but also her level of equanimity) don’t let your shape become an impediment to starting a yoga practice! Or start with a DVD, and practice at home until you feel comfortable.  My original Big Yoga Hatha 1 DVD is out there online, and there’s also Abby Lentz, Megan Garcia, and others who have great DVDs to watch.  If the Big Yoga DVD is too challenging for you, watch for the new DVD, Big Yoga Flex-Ability, a dynamic practice that will help build strength and limber you up  for more challenging yoga.  You don’t have to be thin to enjoy the benefits of yoga! –Meera–from Big Yoga