I can tell you what I know from my experience.
A few years back I heard rumours of a small lake with huge brookies in the Elk Lake region of northern ontario.
Apparantly it was a lake that the mnr stocked and used to entertain certain privelaged bigwigs.
They even built a cabin on the lake to house there guests.
I was told that they'd stopped stocking the lake years ago, tore down the cabin and that there was a remnant population of few but usually large fish.
Of course I had to investigate.
With extreme difficulty I bushwhacked in with camping gear and canoe and the stories turned out to be true.
I camped at the site of the old cabin and caught a dozen or so fish between 4 and 6lbs. Had one on that still haunts me to this day, I figure that fish was 8 plus.
Anyhow, most stocked lakes aren't able to sustain populations of fish when stocking has ended. Without upwelling springs they're simply not able to.
Some stocked lakes like the one I fished have a marginal ability to reproduce. Because the reproduction is so limited the few that do hatch and survive usually grow to enormous sizes. They just don't have the competition for food.