I can't really justify the increasing volume of space I now leave around my sitters as it is entirely based on my gut instinct...
But here's what I think it might be... As I paint the person I'm painting becomes "real" to me and just as in real life, I am aware of and don't invade someone's "personal space."
A couple of years ago, I was working on a portrait and was uncomfortable with it but didn't know why. I couldn't "finish" it. It happened about that time that I attended a party. A very friendly fellow insisted on chatting with me "right in my face"...in fact, he backed me across the room and into a wall (seriously)
Anyhow, when I got back to the studio, I realized that I had done the same thing in my portrait. I repainted it with a LOT more space and felt much better about it.
(Also, I don't know if you can see it on your monitor, but there is a name lettered above her head....)