Originally Posted by
DannoXYZ
Fuji 661 is hi-ten 1020 tubing.
AISI Grade 1020 steel (aka Hi-Ten in bike speak) is one of the lowest cost and weakest of the multitude of available steel grades. It is used in the lowest cost and quality of bike frames and must have thick walls to be adequately strong. So frames made using it will be heavy.
Better and stronger steels include a range of Chromium and Molybdenum containing alloys (aka Cr-Mo) and even more exotic, stronger, alloys which, obviously, are more expensive. These can be made into much thinner walled tubes while maintaining adequate strength so the resulting frames are much lighter.