I don't think you deserve the downvoats, I get what you're saying. As already pointed out, "fit" fatties can be quite strong from carrying their weight around all the time, and can get going quite fast on downhills or flats. Momentum does the rest. Expensive bikes are very light, all else being equal there is less effort needed to move them at the same speed. If you put some muscle behind it, you can overtake them easily, they have a built in speed limit because there's only so fast they can move their legs.