Yep, the last page hints that...
Before the last page, my theory was, that it was the (dark?) spirit of the girl's father, and the pressure was a silent scream of: "What are you doing with my daughter??"
A lot of girls deliberately starve themself to not gain weight. Those who do this during puberty are surprised, that they breast does not grow. The internal structure of the human breast is glandular tissue (for milk production) and fat.
Starving. ➡️ No fat. ➡️ No big tits.