And therein lies your error, I believe. You have assumed that your giant dinosaur has increased in all dimensions by the same amount that its length has increased over a comparative dinosaur, correct?
As a real world example: an earthworm does gain 8 times it's weight growing from 4 to 8 inches - clearly it hasn't doubled in diameter - it has probably only marginally gained in diameter - and thus will be slightly over double the weight. It appears there must be a scale. You, it appears, are using the extreme end of the scale, which would be accurate if two dimensions (I am taking your word for this part) increased at the same rate. But according to the illustrations I have seen, that is obviously not the case. The long, thin parts are longer, but not commensurately thicker.
Re: scaling dinosaurs
Perhaps they were skinnier. But the bones were not. Modern animals with thicker bones have more muscle and skin on those bones than their smaller relatives. I see no reason to think that dinosaurs were different. for the tails being skinnier, at proportionate distances from the main body of the animal the tails and necks are proportionately larger. When you hand a neck vertebrae that would be in the same position on a smaller dinosaur but is twice as tall, thick, and wide then the odds are that the neck in that location is also twice as tall, thick and wide.