Smart girls don't need to ask. If they feel pushing someone's buttons will be beneficial, they will just do it. Most of the time they realize it is not worth the time or effort.
What does pushing buttons gain anybody???
Here's a couple of thoughts.
If the head was bigger it would be heavier and require stronger neck muscles to keep it usable, this would probably unbalance the horse, which is an eating/running machine.
A bigger head would also (probably) be less streamlined, and horses like to run.
I like my horse just the way he is ! I wouldn't want it any other way!