I dont see any signs of light burn or heat stress. Are you sure the light is 600 watts? Is it turned up all the way? What light is it exactly?
It seems to me 600 watts that close should have crisped them, so I am wondering about the light itself.
I suspect there are other issues going on, because they are pretty small, but I doubt its all the lights fault.
When did you start them?
What are they growing in?
What are you feeding them?
How often?
PH?
EC?
Tent temps?
Tent relative humidity?
Air flow?
Lots of things will effect growth rates other than lights.