That's not true - only the tensile strength of steel increases significantly with alloying. All steel is essentially the same when it comes to flexibility. Thus, all other factors being equal, a hi-tensile frame of the same design but using thicker tubing than one built with a thin walled tube will be stiffer (and yes, heavier). If for some reason you built two otherwise identical frames, one with hi-tensile steel and the other with chromoly for example, and used the same tube thicknesses for both, they would weigh the same and you would not notice any difference. Of course that rarely happens in real life as there is no reason to do it outside of a few loaded touring bikes perhaps. But even those are typically butted.