I agree. Sometimes they're dwarf lilies, though (mine are) so they're a bit smaller. I do know someone who has one in a 55 gallon. Also, if you ever have to uproot the lily (for whatever reasons) the roots will take over the tank as well. I had to pull mine up recently to do a massive cleaning job on my 6.6 gallon, and it looked like a freaking monster. The bigger the tank you can put them in, the better.
The aponogeton's aren't too bad, though. I love their crinkled leaves, and the darker shade of green.