Pot might be too small, for them to get large leaves. The plant will stay about the same height until it puts up a flower stalk.

I've got one growing one in a two foot long pot, the leaves are a little smaller than the ones in the garden.

They are in large pots, but that could still be a possibility. I haven't checked the roots, primarily because they are so small. If it has the type of root system that spreads out more than down it would be more likely that that is the case. The pots are about a foot high with a 10 inch diameter, 3 in each pot. The little leaves that are there look healthy, just really small. Many of my neighbors have large healthy looking plants, but those are in the ground. I guess I will have to plant them to really know what is going on.