More or less yes. I like to use the gallon of milk analogy. Regardless of your body weight, which would you rather carry up 10 flights of stairs, a gallon of milk of a pint of milk? IMHO your own body is accustomed to it's own weight. Add something extra to lug around and your body has to work more than it otherwise would have had to do.

That being said, you'd be surprised at how easy it is to shed a few pounds off of a vintage bike without sacrificing durability.