Subject: Is it possible that Dinosaurs were large because of genetic defects?

I was wondering if it is possible that the reason dinosaurs grew so large 
may have been because they had some sort of genetic problem that kept them 
from being able to stop growing in size? And do scientists have any idea 
as to how fast they grew? It seems that even the biggest animals on earth 
(such as whales) take a very long time to reach their size, which is 
dwarfed by the size of certain dinosaurs, such as the brachiosaurus or 
diplodicus. This would suggest that an individual animal could live as 
long as 200 or 300 years before dying of old age. It seems like animals 
that live this long and grow unbounded would be genetically inferior from 
a species of animals that had a relatively shorter life and did not grow 
unbounded.

I once read that when the extinction of the dinosaurs occured, nothing 
over 50 kilograms survived. Is it possible that the reason that mammals 
and smaller organisms survived the meteor impact was because of shorter 
life spans and ability to not grow unbounded because the dinosaurs would 
not have been able to find enough food to sustain themselves after the 
meteor impact blocked out the sun, but the smaller animals didn't need as 
much?

Re: Is it possible that Dinosaurs were large because of genetic defects?