my female cat and her newborn kittens, i need opinions/advice

I have a female stray cat that has been living here for quite some time and she has given birth to two kittens, 3 days ago. One is ginger and the other is a dark grey. The dark grey kitten is fairly small and much skinnier then the ginger. Ginger looks healthy and normal sized although the grey one is twice as small and doesnt look normal size. The mother is feeding them fine but i also have been giving her some cat milk just to help her a little bit more with feeding her kittens. She is also eating just fine so i dont think it has anything to do with her milk. But the mother is a very small built cat so perhaps the grey kitten has turned out to be just like her. They say there is always a "runt" in the family but ive been keeping an eye on them and was thinking of just waiting a few days to see if there is any change in the grey kittens development and if not what should i do? if i wait a few more days to see if there is any difference and if there isnt, should i take over?  

It's possible that she was impregnated at different times by two different male cats.  This would explain why one is smaller.  Keep them warm and weigh them daily.  If you see the smaller one isn't gaining weight or is lagging, having difficulty suckling, or if mom seems disinterested in it, then supplement.