In Scots mickle means small or a little amount of something, except when it doesn’t. As any good Scots grannie knows, “Mony a mickle maks a muckle!”
Mickle is not so often used in modern Scots as muckle. The only time I’ve heard it used is in the phrase “Mony a mickle maks a muckle” and in that context it’s definitely being used to mean small. However, apparently it originally meant the same as muckle. That’s confusing! So, let’s agree on muckle means big and mickle means an amount of something which could be big or small.
I have also heard the theory that the phrase was originally Mony a pickle maks a muckle, with pickle meaning small, and the corruption happening to make more of the alliterative qualities of mickle and muckle.
Whichever version is historically accurate, in common usage (which is not really very common at all) mickle means small.