Some people like their sweaters quite close fitting, and some like lots of room in theirs. As you know from shopping, not all size medium tops are the same size.
The best way to choose which size to make for yourself is to measure a sweater that fits the way you like. The photo below shows how to measure the finished chest of a sweater that fits the way you like.
Pure and Simple patterns have finished chest measurements in the beginning of the pattern. Just choose the size closest to your perfect sweater, and make that size. If you don’t have a sweater that fits you as you like, go shopping, find one, and measure that one.
This is the way you can get the size you want from any pattern, not just KPS patterns.
The children’s sizes are based on commercial clothing found in department stores, leaning toward roomy.