WHAT MUST BE ITS SIZE? FOOD FOR TWO MEALS FOR EACH, THE QUANTITY BEING THE FOOD ONE EATS ON WEEKDAYS AND NOT ON THE SABBATH; SO R. MEIR. R. JUDAH RULED: AS ON THE SABBATH AND NOT AS ON WEEKDAYS. AND BOTH INTENDED TO GIVE THE MORE LENIENT RULING.
Rabbi Meir assuming that because the food is so good on Shabbat, one is tempted to eat more bread – so the weekday amount is the minimum because it is smaller. Rabbi Judah did the opposite, eating less bread on Shabbat in order to save room for all the good Shabbat food! In either case, the smaller quantity is the minimum.
There is quite a lot of discussion then on how quantities are calculated in an era which lacked standardized measurements. For example:
Our Rabbis taught: The Jerusalem se'ah exceeds that of the desert one (described in the Torah) by a sixth, and that of Sepphoris (in the Galilee) exceeds that of Jerusalem by a sixth.
In any case, quantity matters:
HALF OF THIS LOAF IS THE SIZE PRESCRIBED FOR A LEPROUS HOUSE,(Lev. 14:33ff) AND THE HALF OF ITS HALF IS THE SIZE THAT RENDERS ONE'S BODY UNFIT. (Yoma 80b)
If someone stays in a leprous house the amount of time it takes to eat half a loaf, they become ritually impure.
From this it has been inferred that if a person consumes such a quantity of food he is sound in body and happy in mind. He who consumes a greater quantity is a glutton and he who consumes less suffers from bad digestion.