Beitza 17a - Ribuy Shiurim

The gemara in today's daf makes a distinction between cooking extra and baking extra. Really there is no difference between cooking and baking, but rather whether it is done in one shot (i.e. putting a large full pot on the fire) or in multiple shots (sticking loaf after loaf to the oven wall). Really cooking extra should be an issur torah, but the Ran explains that increasing shiurim over what is needed for ochel nefesh is permissible on yom tov, so long as it is done in one shot (to the exclusion of putting a pot on the fire and then putting in piece after piece).

However, acc. to Rashi that mitoch is matir even something which is not a yom tov need, it would seem that the prohibition to cook extra on yom tov would at most be an issur derabonon of hachana for after yom tov (unless you say that cooking for after yom tov is worse than for no reason at all, which would answer tosafos question 12a from pesachim 46b, as pointed out by r' yossi in an earlier comment). It comes out that acc. to Rashi the concept of Ribuy shiurim may only apply to remove an issur derabonon such as preparing from yom tov for chol.

This approach would be very helpful in dealing with the Ran's question from menachos where the gemara seems to imply that cutting a stem with more figs than one needs for pikuach nefesh, is assur min hatorah, why don't we say since it is done in one shot it is Ribuy shiurim and permitted, like we do here?

Acc. to Rashi the answer is that we only rely on this concept to remove an issur derabonon of hachana, but ribuy shiurim would not be matir an issur torah of kotzer. Therefore, Rashi will not have to make a distinction between yom tov and shabbos or between pikuach nefesh and ochel nefesh like the Ran.