Tie down the tops if they get too tall. Stretching is very normal at this stage. Yes, they will fatten out mostly duirng their last 2-3 weeks of flowering, so have patience, you won't be dissapointed.

And DON'T TOP THEM WHILE THEY ARE IN FLOWERING! I don't know who suggested that, but that is a BIG time no-no. You will stress it out and it will turn hermi on you, or there is a really good chance anyways. Just tie the tops down by carefully bending the top stem, but be careful. Cannabis stalks are hollow, and they will break if bent too far.

Good luck.