… You can't really get mammals, it doesn't seem, to be the size of dinosaurs." Mammals lack such air sacs, though, "that can invade the bone and lighten up the bone," Brusatte said, "So elephant size or a little bit bigger, that might be the limit as to where mammals, at least on land" can get.
That's because body weight grows much faster than bone strength as animals increase in size, as physicist Neil deGrasse Tyson has explained.) "The same way that skyscrapers are getting bigger and bigger and bigger because of the internal support structures." (Of course, though air sacs helped make for strong, lightweight bones, no animal could actually get as big as a skyscraper. That helped them get bigger and bigger and bigger," Brusatte said. That gave dinosaurs skeletons that were "still strong and still flexible, but lightweight. In dinosaurs, too, air sacs likely extended from their lungs to their bones, creating sturdy but light scaffolding, Edinburgh University paleontologist Steve Brusatte told Scientific American.
Put another way, as some reptilian youngsters ballooned into hulking adults, they traded their relatively puny juvenile teeth for bigger weapons, allowing them, in turn, to hunt bigger meals to fuel their larger bodies.
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