They also mature more slowly and produce fewer offspring less often than small ones. Smaller creatures makes up for their shorter lifespans by becoming sexually mature VERY quickly and breeding like crazy until something eats them. Larger animals don't have the eating problem, so the fact that they take a few more years to reach maturity means their populations can still remain stable.
The thing is, if something happens that reduces the lifespans of the larger animals, they still produce fewer offspring than they would have under better conditions. What's more, a reduced OVERALL lifespan often means higher mortality rate for juveniles, which can exacerbate the problem (this is exactly what's been happening to elephant populations in Africa for the past 90 years; in their case, the cause is increased human predation due to the development of powerful firearms).
It's not MY arguments, it's just the standard explanation in biological sciences. There's not much debate over that issue, the only question is the details (e.g. WHAT FACTOR is causing the reduced birthrate/increased mortality).
Slightly, yes, through a combination of effects (reduced energy and virility, reduced cross range of individuals looking for mates, slightly longer resting periods and slightly reduced periods of activity). A reduction of just 2% would be significant for a species whose mortality rate was already high, and a reduction in atmospheric oxygen might increase that as well.
Dinosaurs are not, strictly speaking, reptiles.
