Love on A Diet (2001)

Sammi Cheng plays Mimi Mo, a young exchange student to Japan who met and fell in love with a budding pianist, Kurokawa, played by Rikiya Kurokawa. Kurokawa eventually leaves to study music in the USA and returns to Japan as a famous musician. Meanwhile, Mimi has dealt with her depression by eating and weights over 300lbs and Kurokawa doesn't recognize her. Mimi eventually meets Fatty (Andy Lau) who helps her lose the weight so she can fulfill her promise of meeting Kurokawa made 10 years earlier. But then they fall in love.....