It seems strange, but it is not a typical noun. It is the participle of the energetic voice of the Ancient verb ενδιαφέρω, that is ενδιαφέρων, ενδιαφέρουσα, ενδιαφέρον. These words are used in modern Greek too. But in this case it became a noun, keeping conjugation, in the 4 cases, instead of 5 (+dative case) of Ancient Greek.
is there a difference in meaning with λίγος?
Yes, μικρός refers literally to size and λίγος to quantity: μία μικρή σοκολάτα (a small/little (bar of) chocolate) vs λίγη σοκολάτα (little/not much chocolate). As you say, it appears that
you use μικρός in this sense of "little in quantity"
In essence μικρός is like the word 'little' that can be used for either size or quantity, but its use to describe quantity is figurative. It is still valid though and not unusual at all.