The heavier disc is going to have a tougher time staying in the air though as well, the disc is trying to lift, "glide" and stay aloft. A heavier disc could fly shorter then a lighter version in theory. It would be interesting to see which one goes further based on the disc model and speed if throw.
I know my uncle throws some light weight discs and I've tossed a few of them and was surprised how much they glide/stay aloft. I think that the glide might make up for the lack of weight but just maybe only at slower speeds? There could be a crossover point as well. The 70 mph throw might slow down much quicker and the glide can't make up for the initial loss. Where as the 45 mph throw the glide makes up for the slower initial velocity?
Interesting things to think about.
I'm sure everything is different for everyone. I have experimented and I consistently get better distance with heavier discs. I'm kind of a noodle arm so my theory is there isn't a big difference in velocity regardless of the weight of the disc that I'm throwing and the heavier ones have more momentum or inertia or whatever and carry farther. Tailwinds are the one exception where I can get better distance with a lighter disc.