I tend to look at it the opposite way. An LP tank tends to be big, heavy and 2/3's full. Compared by approved capacity, LP120 vs an HP119 for example, the LP120 is 10 lbs heavier and 5 inches longer. Even when you compare tanks with similar dimensions, LP80 vs HP100 for example, the weights are nearly identical
Is that full (of gas) weight or empty?