Q: I'm 200 pounds but most people usually underestimate my weight by 20 pounds . I am not muscular and have a body composition of only 20% so I dont know why I weigh so much considering that I look rather slim and I am of medium frame.

A: It could be for a number of different reasons...

• How tall are you? The taller you are, the heavier you'll be. Every inch of height adds 7-10 or even more pounds to the bodyweight for an adult male. So if you are, say, 6 foot 3 inches then 200 pounds is not much at all.
• 20 pounds one way or the other in somebody else's estimate is not that much, really. You're probably fully clothed when they make that judgement and they are are not necessarily good at estimating somebody else's weight. A 10% error is not that surprising.
• Are you pretty certain of the 20% body fat estimate? Average male has 15-20% bodyfat. You say that you look slim, while I would say that a below average male does not look slim in general. So perhaps that 20% body fat estimate is not correct? Was it taken by a professional trainer using calipers?

I would not recommend worrying too much about your absolute weight or even exact body fat percent. Instead ask yourself if you wish to improve your current physique and if so, how? If you are happy with the way you are, it doesn't really matter how much you weigh or what your BF% is. If you do want to make improvements, then stick around on this site, learn about proper dieting and weighttraing both for muscle gain and fat loss, and ask more questions as they come up. Good luck!