They may have been able to fly because, perhaps, the air pressure was different back then. 2 bar or more
http://levenspiel.com/wp-content/uploads/2016/02/DinosaurW.p...
Going back even further, there's giant insects. Bugs today are limited in size to the efficiency of their respiratory system. They don't have lungs like vertebrates. Air just sorta diffuses through their bodies. I you raise insects in a high oxygen environment, after a few generations they get bigger. The size of bugs before the dinosaurs indicate a higher oxygen content.