Just as an idea, something to toss back and forth: we know George Kirk saved 800 lives. Is it possible that the Kelvin was transporting people at the time she encountered the Narada?
It's own standard crew might be a lot less than that. And if the Kelvin were carrying passengers, perhaps she had a greater than usual number of shuttles too? Perhaps larger ones with increased cargo capacity.
A very good possibility. I'm also rather fond of the idea that there were one or two less armed ships following a bit behind the Kelvin off screen, and that their complement, combined with the Kelvin's, was 800. The Kelvin's sacrifice would be the only thing that allowed them to escape.
Why did Nero attack the Kelvin?
He'd set off on a mission to destroy federation worlds. After going through the wormhole the first thing he sees is a federation ship. He also needs to interrogate someone about the location of Spock, and isn't in the mood for a friendly chat.
Exactly.