That's how they did it. Take a modern skeleton, determine the minimum wrap volume, take known body density figures, that gives a minimum body weight for the animal. But actual measured animals in the natural state weigh in at 20% more than this.
Do the same for a dino skeleton to get the minimum weight. Now assume wild dinos were more like modern animals than roly-poly butterballs, so use the same 20% addition to get natural weight.
They should also weigh emus and ostriches by way of comparison.