I was watching a presentation by Dr. Jessica Theodor  who discusses (at 38:10), among other things, the skeletal structure of dinosaurs vs. mammals. "Dinosaurs have way lighter skeletons, which means that for the amount of material they can be bigger. If we start to look at sauropods, they have this really interesting set of structures, these big holes in their vertabrae... it allows you to build a lighter, stronger skeleton. Mammals can't do that. It's not how we build our bones."
Why don't mammals do this? Are there fundamental trade-offs which make our (apparently) weaker, heavier bones better, or is it just chance that we haven't matched this development?
(Of course I'm open to the possibility that I misunderstood the talk, or that it was simplified to the point of inaccuracy on this point.)
 Jessica Theodor, Being Giant: Why are mammals not as big as dinosaurs? (2015)