Titanium is about half the density of steel, so lighter for the same dimensions, but it also has half the elastic (Young's) modulus of steel, so twice as flexy (though still 50% more stiff than aluminum). This is all elastic flex, no permanent yielding of any of the materials. If the Ti BB feels stiffer, it's because a) (if) the center portion is larger diameter than the steel spindle, and/or b) (if) the bearings are more radially rigid, or impart some bending stiffness to the spindle.