As a famous HSBC commercial depicted, the Europeans believe it's a slur on your host if you don't clean your plate. Whereas Chinese feel you're questioning their generosity if you do it.
Of course nobody tells you that. For us it’s a sign of courtesy (and the result of years of severe education) to finish all what we have in our plate. Unwittingly condemning ourselves to have it re-filled twice, three times, four times, endlessly… until you finally collapse!