Why Aren’t Birds Bigger?
Birds are bigger than mammals, but the largest-ever bird weighed less than a cow. Why aren't birds bigger? One theory is that as avian eggs get bigger their shells get disproportionately thicker to support their weight. Eventually the shell would be too thick for a chick to break through, and that’s your maximum egg size (and thus maximum bird size). I don’t buy this.
Status: Initial allometry of ratite shell thickness done.