Add body weight and the old gear sums to about three percent heavier than the modern gear. I'd say total weight matters more than gear weight alone, doesn't it?
Anyway, you can try it yourself, wear a 2kg wax cotton jacket versus a 500gm technical jacket and see how you feel after a day's hiking.
Carrying your lunch on a 10-foot pole, keeping it off the ground at all times, versus slipping it into a fanny pack - or eating it and carrying it in your very center of mass.
I noticed while ultralight hiking (full kit without food, fuel, and water under 9 lbs, for multi-day excursions) that how close your backpack was to your back mattered. Unfortunately, if it was tight to your back it overheated you, so a standoff of an inch or so was essential. I considered dividing it front and back, so each was about half as "thick" (far from my body), but there isn't a lot you can carry in front of you without seriously impeding movement.
Anyway: force times distance equals work.
Two kilograms extra is gigantic.
If you have a friend who hikes or backpacks, ask them to take you along for your first time and try it out for yourself.