You have to take a more holistic approach. In days of old a 8-9kg bike was considered reasonably light, but even the there were a few that got below 8kg - even with steel frames. Knew a lot, those people at Reynolds and Columbus. There were builders who used techniques familiar today that were advanced, lugless frames fully welded, and alos aluminium frames (Alan being a good example). Generally almost everyone was on the same level playing field. Down tube levers and handlebar end controls weighed little more than integrated levers do today, in fact brake levers were probably lighter. Seven speed blocks (screw on, none of your new-fangled cassettes!) were even then being made lighter, but a freewheel body in steel was a hefty lump, and all bikes were tail heavy, or at least more so than today. Gearing was changed stage by stage, 42x25, 39x24 were common bottom end ratios, although for really steep climbs up to 28 sprockets could be fitted. it just spaced out the gearing, so you might end up with 53x13 to 39 (or a 38 if the mechanic had good contacts at a manufacturer)x28, It got over most things. There were still some 20%+ bits used even then.
So overall, the answer is that kit has got a bit lighter, ratios are closer spaced and go as low as 34x29 (or even 30) as this lets riders spin better, which reflects the technique of faster pedalling versus a bit of brute force. Some riders still use bigger gears today. The weight thing overall is more to do with body weight so the overall package is light, then we have to consider bottles (a couple of full 500ml. ones add a whole kg to weight). Riders have the same spectrum of ability now as always was, and a super light bike (minimum today 6.8kg, but I bet that limit will be reduced) still won't turn a donkey into a race horse.
A couple of bits of information I picked up from reading and nattering the other day to a mate who is now an official:
The heaviest bike at ToB this year was a TT special (for the last but one stage in London) with disk wheel which weighed in at a whole 9.3kg.
The bike used by Matt Clinton at hill climb champs (run by CTT who don't care about UCI rules) was pared down to 5.2kg. Which might make anyone think that UCI limit might be safe at 6.2kg or a bit less without having bikes break all over the place.
I wouldn't want to ride a 5.2kg bike over the pave or any poorly surfaced road though!
