The bees that are taking syrup from the feeder seem to be mid-sized bees, smaller than the older foragers. Since I still see foragers bringing back some pollen I'm hopeful that it's the house and receiver bees that are pulling from the feeders. I wouldn't want the foragers super distracted with local feeders.
But, along that same line of thought. If we ever had an open feeder in the field I imagine it would be 100% forager bees that were gathering the sugar water.
When your bees hatch they are the size that they are going to be the rest of their lives. They would have to shed their skin to grow because they have an exoskeleton. The size of the cell is the major determining factor of the size the bees will be.
Forager bees are the ones that you will see in an open feeder.