Why do J-girls inflate their cheeks when taking pictures?

Sachiko Wantanabe of Hammamatsu, Shizuoaka was the first girl to inflate her cheeks when being photographed. This occured on March 23, 1991 at a downtown purikura booth. She was 17 at the time. Ms. Wantanabe, now a mother of two related in an 2009 Sankei Shimbun interview, "I just wanted to look cute, not start a trend".