Bluegill most likely but could also be RES or some RES could be at the margins. Below is a picture of a half dozen RES nests in my one acre RES pond that has no BG.
My RES nesting areas are in groups around the pond in different places because I do not have one big area like you have in your picture where so many can be together.
I also have some similar looking nests scattered on underwater humps I built that I would guess are SMB but they also could be RES. Hard to tell without seeing the fish on them and I can only see them when the water is low and the fish have left.
If you look closely in the last picture you can see a RES warily watching me wanting to get back on the nest (dark spot on the far side of the center circle). These pictures were taken a few weeks ago right around a full moon. About 4 RES were on nests that I could tell.
As far as the size of the nest keep in mind fish will clean out old nests and re-use them from previous spawns. I looked at the above nests just yesterday and they had no fish on them but some loose dead FA has settled in the middle. I fully expect these nests to be cleaned out and used over and over as time goes by.
