Well, they took inspiration from Clement Clarke Moore's (1922) "Twas the night before Christmas" and illustrations from Thomas Nast (1881) showing him in red with the sack over his shoulder. Certainly, Coca Cola has solidified the modern image with their ad campaign though starting in 1931. Src
Mass per volume is density, and I suspect the Trex and kangaroo are similar (~1000 kg/m^3) so yes they'd both bounce in that case. I think what you're looking for is surface area to volume, which decreases rapidly as an object gets larger while maintaining geometry.
My bad. I was thinking of Musk