They're not "even bigger", they're structured and regulated very differently now.
In the US, we can't just convict people because they lost people money. You need to prove criminal intent. Are you aware of any evidence demonstrating intent to commit fraud?
Not sure what the size of banks has to do with anything.
I dont have detailed knowledge, but rating agencies clearly violated their duties by providing AAA rating to junk
Any evidence that specific executives at these rating agencies had the intent to defraud people by over-rating mortgage-backed securities? Simply being inept at your job is not necessarily criminal.