The HK91 is not a light rifle and its balance is attributable to its massive wolfram filled bolt carrier which tends to make it top heavy. If you ever get your hands on a bolt carrier, shake it next to your ear and you'll hear the grains as they shake up and down. It's put in there to help decelerate the bolt carrier's travel.
They're not really much different from the FN-FAL or M14 in weight, but their bulky triangular forearm and the balance makes them feel heavier than either of the other arms. If you get a chance, try one with the slimmer forearm and I'm sure you'll agree that it feels much better (the triangular jungle forearm is suppose to be better for rapid cooling).
Why were they popular? German engineering, reliable and accurate. They weren't that expensive in the old days ($350-$400 around 1978-80) and were the cats' meow when it came to black rifles. They were also easier to convert to sharpshooting rifles (Blake Stevens in his monumental work on the FN-FAL reports that FN gave up on trying to make a sniper version of FN) and will hold together longer than the M21 (M14 version of the sniper rifle).
Like Spartacus says, I love both of them but if I had to buy only one, it'd be the AR (see posting on Steyr Aug for my reasoning behind that).