Bees have an economy. It depends on available resources in the hive, resources coming in and workforce both to gather those resources and to maintain the heat, humidity and safety of the hive. In other words they have to guard the combs from ants, wax moths, small hive beetles etc.
Once the overhead of maintaining a hive is met, then there is a surplus of labor to haul in a crop for winter, build more comb, clean the hive etc.
So the results of this are that a hive hits, what is commonly called "critical mass" after which things build quickly and before which things do not build quickly. Ideally for the purpose of increase, you want a split that has "critical mass" from the start so they will build quickly and put away a crop for winter. This is usually about one deep full of bees and brood and stores or two eight frame mediums full of bees and brood and stores. Any split smaller than this will struggle for a time before they really take off. I make a lot of nucs smaller for purposes such as banking a queen, or mating queens, but I don't intend for them to build up enough for winter.http://www.bushfarms.com/beessplits.htm#earlysplit