I think the big rain and then the cold snap in May screwed up the bluegill spawn. They were just going on bed and then the rain and cold hit and scattered them.
I always see a few "straggler" bluegill here and there bedding on the full moon in June and July, but this year there are more bedding bluegill than I usually see. Yesterday I fished up in Spencer Creek on Old Hickory and I came across two or three very large expanses of spawning bluegills. I even found a group of bedding shellcracker.
Most of them were relatively small though - same as your experience. Maybe the bigger fish went ahead and spawned out by now and the smaller fish are still catching up, I don't know.
bd