Ijsselmeer is a different kind of lake; not created by digging out peat, but a (violent) sea inlet that was closed off. The many peat lakes didn't start out as strips of land, but as endless peat swamp. Much of that was drained through drainage ditches and turned into agricultural land. Other parts were dug up for fuel, creating those massive lakes. In the latter case, they sometimes left those strips of land to transport the fuel.