As far as I have read plant experience serious growth in the 2-4th weeks of flowering (in general, varies by strain), then in the last few weeks they stop growing and concentrate on Bud production. I would say you're ok. I'm using pots around the same size and have noticed slow growth but it's ok since I don't have much more space. Repotting now may stress the plants, but if you're really worried try one (i wouldn't). Just be patient.... I'm dying as I'm just in the 2nd week of flower!!