It was once believed that bluegill could not be fished out; now that idea has been thoroughly disproven by biologists in multiple states, as well as by professional pond and lake managers in the private sector. Biologists in Illinios and Utah, among other places, have discovered the existence of two discrete classes of male bluegill in any population, parental males and sneaker males. Parental males mature sexually at much larger sizes than sneakers, which can mature at three or four inches in size. Sneaker males, because they have not developed the body characteristics of larger males such as a large opercular tab, maroon or orange breast, muscular forehead, etc., are able to fool parental males guarding nests during spawning season into thinking the sneaker males are females - the parental males let them onto the nests, the sneaker males fertilize the eggs with their inferior genetics, and the overall genetic profile has just declined in that bluegill population. When there are a large number of parental (larger) males in a population, though they are duped by the sneaker males some of the time, they still fertilize a good number of the eggs themselves, and run off the sneaker males much of the time...
Unless misinformed, or greedy, fishermen keep too many of the large males, in which instance the sneaker males have free reign, which can cause the genetic profile of an entire population to permanently decline.
I have fourteen years of experience managing private ponds and lakes, and over thirty years of experience fishing and closely observing public lakes, and I've seen this very thing happen many times, most recently three years ago on a 50-acre lake near my hometown. The lake had never had exceptional bluegill fishing, but suddenly one summer I caught a couple decent ones, and resolved to return the following spring; sure enough, I had two or three trips where I caught a dozen or more large bluegill each time, some of them in the pound range. This was in March; I remember that at one point the "fishing report" page on the website maintained by the people that run the concession, remarked that they couldn't believe how many big bluegill were being caught. Within the span of two months, the meathogs had done irreversible damage to the lake: the fishing report page on the website lamented how scarce the bluegill were that year (they had already forgotten how many had been yanked out two months earlier)...And the lake hasn't had big bluegill since. I saw the same thing twenty years ago on another public lake about thirty miles east of me, same song: huge 'gills appeared in the lake four years after being drained and restocked, the word got out and the meat fishermen showed up, and within a year the lake's big bluegill had been decimated; today the bluegill fishing in it is pathetic.
One may think that a large reservoir can't be fished out, and the small bluegill can't. But the large ones can just as surely as can large bass. All one has to do to confirm this for oneself is to do a little research on what the bluegill fishing was like thirty years ago on any of thousands of once-legendary 'gill lakes across the country, compared to what it is on those lakes now. It's not coincidence that multiple posters on this thread noted that they used to catch bigger bluegill when they were growing up than they do now: this can be traced directly to the fact that the understanding of bluegill anglers in this country regarding their quarry is about forty years behind what bass fishermen have for theirs. There are many times more fishermen plying public waters now than there were even just thirty years ago, more people yanking out big bluegill, more harm to the gene pools of each lake. It used to be the norm for bass fishermen to keep big bass without thinking twice, and lakes got fished out; catch-and-release came along, and bass fishing in public lakes rebounded across the country. I and others who take bluegill fishing seriously, and pursue this wonderful fish year-round rather than just when they're easy pickings on the beds, hope that someday the level of angler awareness and enlightenment about our favorite fish, reaches the level it long ago did for bass, or walleye, or muskie, or pike, or trout.
Here are a couple photos of what bluegill can look like when the genetics of a population are not decimated by overharvest (other factors also went into these fish, to be fair, but they never would have reached this size had they been yanked off beds at age two):