>>38238394It's based off the canon boob chart that the mangaka jokingly answered for a fan during the 20th anniversary meetup.
So basically, he described all of the girls' chests and what made them different from each other.
Like Blue/Green used to be queen of biggest boobs for Spe girls, but then Y took the cake and it says stuff like her boobs are more rigid and less squishy because she works out a lot etc.