Comb built in the spring and during a honey flow tends to be made with worker-sized cells. When built later on in the summer and when the nectar isn't pouring in, the bees tend to build drone cells.
As for a reason, I can suggest one but can't guarantee that it is the only reason.
Drone cells, being larger, use less wax per unit area. When not much nectar is coming in, the bees produce less wax and by building larger cells they can make the same amount of storage room with less building material. When they are producing large amounts of wax they can afford to be more liberal in their use of the wax for construction. Another consideration is that in the spring time, the hive tends to have a larger population of young workers, and they, in their youth, tend to exude more wax, enabling the construction of the smaller, worker cells.
Using foundation with small cells can push the bees to build worker cells in circumstances when they would on their own, teend to build drone-sized cells.