Reference for Bava Metzia 6:5
הַשּׂוֹכֵר אֶת הַחֲמוֹר לְהָבִיא עָלֶיהָ חִטִּים וְהֵבִיא עָלֶיהָ שְׂעֹרִים, חַיָּב. תְּבוּאָה וְהֵבִיא עָלֶיהָ תֶבֶן, חַיָּב, מִפְּנֵי שֶׁהַנֶּפַח קָשֶׁה לַמַּשְּׂאוֹי. לְהָבִיא לֶתֶךְ חִטִּים וְהֵבִיא לֶתֶךְ שְׂעֹרִים, פָּטוּר. וְאִם הוֹסִיף עַל מַשָּׂאוֹ, חַיָּב. וְכַמָּה יוֹסִיף עַל מַשָּׂאוֹ וִיהֵא חַיָּב. סוּמְכוֹס אוֹמֵר מִשּׁוּם רַבִּי מֵאִיר, סְאָה לְגָמָל, שְׁלשָׁה קַבִּין לַחֲמוֹר:
If one hires an ass to carry wheat and it carried barley, [which is lighter than wheat], he is liable [for damages if he added three kabin. We do not say that since barley is lighter, he can add until the weight of a letech of wheat (the carrying weight of an ass)]. (If one hires an ass to carry) grain and it carried straw, he is liable. For "volume" is as difficult [for an animal to carry] as a burden. [Even though it does not weigh as much as wheat, its volume is the same, and volume is like weight.] To carry a letech of wheat (half a kor. A kor is thirty sa'ah], and it carried a letech of barley, he is not liable. And if he added to its burden, he is liable. How much must he add to its burden to be liable? Somchos says in the name of R. Meir: A sa'ah to a camel; three kabin to an ass. [The halachah is in accordance with Somchos. If he added less than this amount, he is not liable if the ass were injured, and he pays for the additional amount alone.]