Aluminum has a yield strength very close to its ultimate strength. In other words, it is quite brittle, and prone to breaking. It just doesn't take bending very well at all. That's why aluminum frames are built with such large tubes. The frame cannot be permitted to bend to any degree. Those extra big aluminum tubes also provide the "stiff" aluminum ride. While steel is actually much stiffer than aluminum, it has an ultimate strength much greater than it's yield strength. So, steel tubes can be made very thin and small, allowing the entire frame to provide some flexing, thus producing the "steel" ride qualities.