Extending the prior replies, you are, in effect, sailing with your first reef in. The result is a bit less power, a slightly lower, slightly more forward center of effort. If you put a full sized main on, you will note that the balance between sails changes to give you a bit more windward helm, which is easily compensated for by the way you trim your main. If short, take your undersized mainsail and go sailing. Have fun.
Jim Kolstoe, h23 Kara's Boo