Yeah but chromoly is much stronger, is it not? So it's stiffer in tubing. I think I read that it was about 4 times more expensive which I take to be the reason why Brompton like hi-ten:
"High-Tensile (Hi-Ten) Steel
High-tensile or carbon steel is a common and inexpensive alloy comprised of iron mixed with 0.2% to 2.0% carbon. (Of note, when iron is mixed with more than 2.1% carbon it is no longer "steel", it goes by the name "cast iron".)
High-tensile steel is an inexpensive but relatively weak alloy. Although it has essentially the same density as other steels, manufacturers working with hi-ten steel are forced to use thick walled tubes to ensure adequate strength, and rarely use butting. For this reason, high-tensile frames are much heavier than their chromoly counterparts.
Today, high-tensile steel frames are primarily used for children's bikes and are sometimes found on inexpensive adult bikes."