That's because they think yoga is an actual workout.  Yes, it has many benefits, but the mind/body connection is the main goal of yoga and it has more to do with bringing you peace and comfort than fitness results.  It is a great addition to, not a replacement for, a fitness regimen.    Fats see yoga and think "yay, easy workout!  I will be thin in no time!  Teehee!" ETA: I am a personal trainer and do not mean to imply that yoga is actually easy when done correctly.  I know it isn't and also know it is a real workout for serious practitioners.  Fats always do the laziest yoga possible and cheat on all the difficult poses, so to them it is easy since they just don't bother doing it.  Laying on a mat for 45 minutes is easy and if they do Bikram yoga they can convince themselves they worked up a sweat too.  God I hate fats so much.