פירוש על בבא מציעא 3:7

המפקיד פירות אצל חבירו – and that which was deposited was combined with his produce and he was doubtful from them and did not know the measure of what he had consumed. And when he comes to restore them, he should take out the loss, deducting what they normally deduct [due to the produce 3consumed by mice].

When a man leaves produce for his fellow, [and his fellow returns it to the owner] he may exact reductions [due to natural depletion of the produce]. For wheat and rice, nine half kabs to the kor. For barley and durra nine kabs to the kor. For spelt and linseed three seahs to the kor.
All is in proportion to the quantity and according to the time [it is left]. Rabbi Yochanan ben Nuri said: “But what do the mice care about [the quantity]? Won’t they eat [the same amount] whether there is a lot or a little? Rather, he may exact a reduction from only one kor.
Rabbi Judah says: “If the quantity was great he may not exact any reduction, since the produce increases in bulk.”
Mishnayoth seven and eight deal with the reductions in the quantity of certain deposited products (produce, wine, olive oil etc.) that a guardian is permitted to make when he returns the products to their owner.
In our mishnah Reuven left his produce with Shimon and is now coming back to reclaim it. Shimon need not return exactly that amount that Reuven left but is legally permitted to reduce some of the quantity, on the assumption that mice would have eaten some of the produce. The mishnah then proceeds to list how much can be reduced for different types of produce.
According to the anonymous opinion in section two the amounts listed in section one can be reduced for each kor (a measure of volume) for each year that the produce was left with Reuven. Rabbi Yochanan ben Nuri raises a difficulty on this opinion. He points out that the mice will eat the same amount whether there is a lot or a little. In other words if there are 10 kor or 1 kor the mice will still eat only nine half kabs of wheat or rice. Therefore, according to Rabbi Yochanan ben Nuri no matter how many kabs of produce there are, Shimon may only reduce from one of them.
Rabbi Judah states that if the amount of produce was large than Shimon may not reduce at all. According to Rabbi Judah, large amounts are measured in heaping imprecise measurements. Therefore the extra that was heaped on top of a strict measurement is enough to compensate for the loss to the mice.

תשעה חצאי קבין לכור – A Kor = 30 Seah and Seah = 6 Kabim.

הכל לפי הזמן – As such for each and every Kor.

הכל לפי הזמן – for each and every year he should duct for him such-an-amount

מה איכפת להו לעכברים – Such-[an-amount] they eat from a small portion just like from a large portion. Therefore, nine and one-half Kabim per year whether from a Kor or from ten Korim.

אם היתה מדה מרובה – that he deposited with him a lot, from ten korim and upwards, he should take out the losses.

מפני שהן מותירות – for during the days of the granary when they deposit the wheat which is dry, and in the rainy days, when they return it, it blows up. And regarding the consumption by mice, the loss is not so great for each Kor, the mice do not eat all that much from the ten Korim. Therefore, their blowing-up [in the rainy season], restores what the mice consume. And the Halakha is not according to Rabbi Yehuda nor according to Rabbi Yohanan ben Nuri. And all of these measures are [what applies] in the Land of Israel and during the days of the Tannaim, but in other lands at these times, all is dependent upon what the seeds normally lose in that particular country and time.