Maybe dino-size was related to genome size. There is a (weak) correlation in some tetrapod groups - including birds. [1] [1] Gregory, T.R. 2002. Genome size and developmental parameters in the homeothermic vertebrates. Genome 45: 833–838 http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?issn=1480-3321&volume=45&issue=5&startPage=833 []s, Roberto Takata