Sorry for rude title, but yeah I am playing CKII at the moment and I want to know why this is happening. Why the bar isn't filling up all the way. It's been like this for years.
Conversely, when attacking someone it's now best to attack the realms ruler's personal holdings. So the new system creates men out of thin air for your vassals - those men are both fighting battles on the field and defending their holdings at the same time. Your vassal's levies are twice as effective as those from your personal holdings. I wish they would fix this.