Both began relativly small (think rat and chicken sizes) but dinosaurs rapidly developed increased size. As the predators got bigger to better kill their prey the herbavores got bigger so they could not be easily eaten. It became a biological arms race. Mammals, on the other hand, actually evolved first (in the late permian, evidence suggests) but stayed small to avoid predation. Big predators (and thats most of them) wouldnt bother with such small morsels. These rat to possum sized critters spent their time evolveing new ways to mass produce themselves. These were first the marsupials and then the placental mammals. Being so big and heavily specialized the dinosaurs couldnt survive the next big extinction. The various mammals, being fast reproducing opportunists, reproduced like mad as soon as the catastrophy was over. Soon they and birds were the domanant vertabrates on the planet. Mammals diversified into every specialized niche left open but they did one unusual thing for life. One massive branch of the phylum for some reason remained opportunists despite the short term benifts of specialization. Thus we still have opossums, rats, baboons, men and many other generalists right along side with specialists of the same phylum. Normally in a long period without any mass extinctions nearly all branches of a phylum become specialists, taking advantage of a short term benifit but then becoming trapped in that role. This did not happen with mammals however.
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You know if Troodon was given a couple of million year more then maybe people would be dinosaurs and not mammals think about it!
