Hi,

Steel comes in lots of different strength grades but all grades
are equally stiff, so really good steel allows you to make strong
lightweight flexible frames, cheaper heavier frames are stiffer.

Titanium comes in grades about the same strength as steel,
is about half the density, but is about half as stiff as steel.

However frame stiffness basically depends on tube diameter,
whilst strength is more about wall thickness, but affected
also by tube diameter to a lesser extent.

Basically a titanium frame built with bigger tubes to feel
like a steel frame will end up weighting less than steel.

Same sort of thing with even bigger tubes and thicker
tubes for aluminium alloy frames, as its 3 times less
dense, a 1/3 a strong and a 1/3 as stiff as steel.
Still it makes a lighter bike with bigger/thicker tubing.

rgds, sreten.