so why not baga biodach
I'm trying to figure that one out too. It sounds redundant unless the bìodach part means "really" or some equivalent.
I guess it might just be being used as an intensifier, like "a tiny little bag" in English?
It would be common in Scotland to refer to a very small bag as "a tiny little bag" or "a little wee bag". Many Scottish turns of phrase are direct translations of the Gaelic.