I wouldn't worry! They really aren't too small for Day 15! I'm growing 3 plants of one strain that didn't do ANY stretching whatsoever until they went into flower! Each strain has different characteristics and you just need to go with the flow... as long as they're increasing in height a little bit (i.e., not stunted), you should be fine as long as there aren't any other issues (discoloration of leaves, thrips, etc). I hope you've increased the temps a bit... and I'd try to increase the humidity as well.