I haven't made very many patterns, but of the ones I've made, the finished product is always a bit smaller than what I had imagined, even when I make seam allowances on my patterns. I guess I should just make them even a bit larger when I draw them! Making clothes can be tricky too. I find it helps to use paper towels at first for clothing patterns, because they drape and fold more easily than paper. Once you are happy with it you can trace the paper towel pattern onto a more permanent paper or other material!
Hope that helped a little