Butthurt explanation! Probably due to the surprised or embarrassed look in RD's eyes, along with the slight blush on her face, decrease in overall size and increase in a little bit of pudginess. And I, too, fin it ADORABLE!
I use the basic brush in SAI in normal and multiply modes. I've made a small tutorial on my way of drawing hair, but it's in Russian. I think you can understand the basic idea from the pictures, though. [link]