One has to remember that when you do fill it up, the bike is as light as it'll ever be just prior to the refill. You know that it's there and you use a small amount of caution.
I had a 42 Lt steel tank on my previous R100CS and a 39 Litre steel tank on my R80 G/S before that, both were Ernst Hoske made in Germany. Since 1982 I've been running large tanks and wouldn't go back to standard ever again.
It goes like this:- the 1st bar disappears about 250 - 350 Klm's depending how it's ridden, that tells you that you are getting close to half empty, so I take it from there.
Fully loaded and travelling two up through sandy territory, about 500 K's is safe maximum but it should get about 580. Solo and light work 700 is pretty much tops but I once went 775 Klm's and it was still running, just.
As for weight difference I don't think that there was that much in it, remember that it is bigger, bulk, even though it's plastic, means weight.
Mick.