Short answer is because its more efficient. The long answer is: Along the leading edge of an airfoil (ie a sail) there is upwash, and along the trailing edge there is downwash. Upwash is when a foil causes the flow approaching it to bent upward (or to winward, kind of like a lift), and downwash is when the flow is bent downward (or to leeward, kind of like a header) The mainsail operates in the downwash of the headsail, which is effectively heading the main, making less power. This can be seen when the jib backwinds the main. The headsail on the other hand, operates in the upwash of the mainsail, which effectively give the jib a lift, and makes more power. So, because the headsail is in a more efficient location, if we make that sail bigger, we gain more than by making the main bigger.
