Well, it's a seasonal quote describing the pleasantness of autumn, and the propensity for people binging on food. The Koreans also have a similar expression with pretty much the same meaning.
Ironically, the quote is derived from a Han dynasty Classical Chinese expression (with a Tang dynasty poem as intermediary) warning people of the impending seasonal Xiongnu (nomadic people similar to the Huns) raids whose horses have grown stout on grass and are geared for war.