N.J.
Interesting question!
I'm petite at 5'0 and 102lbs. With both my pregnancies my doctor told me 20 pounds was fine, she actually said I shouldn't gain anymore than 20 lbs. In my opinion 25 - 30 pounds is just a safe number that will ensure that M. is eating enough calories per day to ensure fetal development. Someone who is overweight probably consumes more calories a day then someone who is not overweight, so they don't need to increase their calorie intake much because its already above average. I eat a lot less than my husband who weighs 170lbs. I think its really an individual thing that really depends on a person's overall health. Also, remember that fat is also needed to help produce milk after the baby is born.
~N.