Your plants have probably just finished or are pretty close to finishing the stretching that they do during the first month or so of flower. They have probably nearly doubled in size from what they were when you put them in flower, right? That's where they have been directing their energy. When they slow down on the stretch, that's when they will really start pumping up the buds. Though there are some differences from strain to strain, I think you'll really see that begin in weeks 6-7. You'll almost be able to watch as those buds get fat and frosty.
One thing that you might have done is start giving your plants nutes according to the flowering regimen as soon as you flipped to 12/12. Lots of people continue with the veg nute strength for the first couple of weeks of flower because mj plants do so much stretching early in flower. They kinda transition themselves into flower and they like having those veg nutes early on. If you started them on flower nutes when you flipped to 12/12, that can have something to do with the bud size now, but if you did, don't worry about it now. Your buds will catch up if you are doing everything else right.
