Oh, let me explain.
Butter has a low melting temperature so when you put the cookies in the oven, the heat melts the butter quickly and the cookie dough spreads out thin and makes a crispy cookie.
You can prevent this by thoroughly chilling the dough and the cookie sheets before putting them in the oven. Keep the dough in the fridge between batches and chill the cookie sheets too.
This way the dough has a chance to set for a plumper cookie before the butter melts completely.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan