birds as large as pterosaurs

[Betty Cunningham <bettyc@flyinggoat.com>]

It just occured to me to wonder--why are the largest flying pterosaurs
so much larger than the largest flying birds? 
 Are there structural limits in bird anatomy that weren't present in
theropods to limit size?  
Is perhaps the actual weight of the feather body-covering a limiting
factor? (seeing as the largest of pterosaurs could have been naked
entirely and still capable of flight and not so with birds).
For that matter, why don't we see bats as large as teh largest
pterosaurs?  Is the weight of the fingers in the wingspan again a
possible contributing factor?

-Betty Cunningham