Pretty tall plants for a 600w so that may play a part in it. Plus they look a bit stretched so each branch isn't going to be one long bud. But the plus side is every part is getting light to it so no shading of the smaller buds going on.
They look a bit high in nitrogen which may mean smaller buds, the darkness could be strain related though, I havnt grown those strains. Not so heavy on N that they are toxic or anything though. Tiny bit of nutrient burn.
If your 6 week from flip then your probably 5 weeks along or maybe less. I wouldn't worry, they will get bigger and will defo pack some weight on over the next 3-4 weeks.
All in all looking nice dude. All those buds will add up to a nice crop. Just be careful not to chop them early. Its easy to think they are ready when they are nearly there.
Top work grower.