I finished my first sweater (simple tank) in Blue Sky Alpaca. I mattress stitched both sides

and am now blocking it. (I learned later that you're supposed to block before you sew.) Anyway, my sweater fit well before blocking and now it looks huge. Do things get bigger when you block? Is it just an illusion because it is wet and laid out straight? Help! Should I do something else while it is still wet?