If it holds two kur is it a kli? (Shabbos 35a)

The gemara says that according to Rabba a kli is considered muktza only if it holds at least 3 kur it's muktze on Shabbos then it's a kli but less than that is not a kli. The gemara then quotes Abaye who says that Rabba paskened for him that two kur was not a kli. The gemara does not seem bothered by this apparent stira but quotes a mishna that supports Rabba's view. The mishna in keilim says that it can't become tamei if it holds at least two kurim. The gemara just ends with that not explaining how Rabba and Rav Yosef could learn differently if there is a mishna that says two kurim is not a kli.

I thought that it's possible that for tuma we are not interested in whether something is a kli or not. We want to know if it's metaltel malei. Something that holds two kur is not so therefore the mishna in keilim says that it can't become tamei. However, for hilchos Shabbos we want to know the definition of a kli and for that Rabba and Rav Yosef say that it's considered a kli (even if it can't be moved full) as long as it holds less than three or four kur. This still doesn't answer the stira within Rabba himself. I guess you have to say that he changed his mind at some point.

The Rambam paskens in hilchos keilim like the mishna that more than 2 kur can't become tamei but in hilchos Shabbos he says that size doesn't matter. The only thing that matters is if it is a kli so even if you need 10 people to move it, it's not muktza if it's a kli. The Magid Mishna points out that the Rambam actually goes even beyond even Rabba and Rav Yosef and paskens like another gemara later in the maseches.