Not necessarily true. I weighed the headlamps a while back and if memory serves me well because I do not have my notes in front of me, they were like about a pound, pound and a half between both. So as far as weight, it will be about 1/2 to 1/4 pound shavings.
Glass lights are not that heavy, but I'm a turd for ounces, so I'll count them in anyway. I think I shaved more by going with the A-990 grill facelift because two headlamps were terminated all together and my now singles have the high and low beam feature.