Well, you probably look better sitting on a bike than you do sitting in front of a television. Don't let your perceptions of what other people might think ruin your life. You want to ride a bike, just do it. Have you been riding the mountain bike?
There are other "big" girls on bikes for sure! road bikes, mountain bikes. Whichever bike, just get on it and ride it. It took me a month to get used to my first road bike, but I am sure it is different for each one of us.
Welcome to TE

RIDE!