Mine have stayed the same size since I bought them! Actually, none of my nuc hives have expanded into full hives, is what I should say.
Simple, innit! Add more bees, some proper sized frames, add a little emerging brood, then some more as they emerge. Hey presto, give the bees enough resources to become a truly viable unit/colony and it will be a nuc in a quite short time, say about a further (four?) weeks should do it (as it will need three weeks before her own offspring from the 'proper' frames emerge). Depends, of course, what you consider to be a nuc I suppose....