The maximum depth is probably a worse measure than the average depth of the lake, though. A reasonably shallow lake might have a miniature abyss, which has a greater maximum depth than another lake which is uniformly quite deep. Then which lake is bigger? The intuitive answer is the one with the uniformly larger depth, even though stats like "maximum depth" will make it sound like the first lake is bigger!

Besides, didn't they pay any attention to the name?

See, I'm pretty sure we were taught that Superior was the biggest of the five, which was one of those few geographical factoids which was easy to remember!