One Small Flask

A poster on a discussion list asked why the flask of oil of the neis of chanukah is referred to as a kad katan, being that the volume of 3.5 log that was necessary to fill the 7 lamps of the menorah each night is approximately equal to 1.5 L, hardly a small k'li. One respondent suggested that on an entire mountaintop, a soda bottle would, indeed, be considered small and easy to miss.

I don't understand the basis for this question, as Shabbos 21b (as well as Meg. Ta'anis) merely refers to it as a "pach", with no adjective attached to it.

Is there an alternative source on the matter (besides our collective memories of kindergarten)?