If he hadn't been angry and irritated, he might have noticed that happened. Or rather, he would have noticed AND taken in to account that their little tiff could get ugly very quickly.

But he didn't, and he just snarled back at the smaller girl.

"Loudmouth? Please! I wasn't the one who started a conversation by yelling out 'hey Space Brain!'! If anyone is the loudmouth here it's YOU! ... shorty!"

The last part, of course, added in just to be a further thorn in her side.