פירוש על ברכות 106:23

A SIZABLE MEAL. Resh Lokish mentioned two different periods of time that are needed for digestion of food, which is the time until one can still recite birchas hamozon. The Gemara distinguishes between a large meal and a small meal: For a large meal, the time required for digestion is the time it takes to walk four mil.¹Four mil is approximately four thousand feet. According to most commonly accepted calculations, it is seventy two minutes. This is Rashi’s explanation. And for a small meal the amount of time for digestion according to Resh Lokish, is as long as one is still thirsty.
And it appears to me that four mil is just a bit more than the time one remains thirsty. It is unlikely that the Sages would establish for digestion two periods that are so close to each other. Rather, it appears that one should explain the Gemara the opposite way: for a large meal the time required for digestion is as long as he is still thirsty and there is no fixed amount of time. But for a small meal, the time it takes to walk four mil is needed for digestion.

In the opinion of Rashi, this means a walk of four Mil for a heavy meal; but Tosafot explain four Mil for a light meal.