Why is it petty things like petty theft makes me more angry than certain bigger crimes?

That's an interesting question, and I myself can relate. My opinion of why it's so irritating is simply that the perpetrators are potentially upsetting someone for such a tiny gain. They are committing ethically wrong acts, and for what? Maybe part of it is that the petty thefts never get caught, that the police can take more action. Personally I think the reason lies in the immorality of those who steal. It seems to me that anyone committing an unethical act, (even if it doesn't exactly hurt someone it's still wrong), must have a messed-up moral comp if they get such little gain. Interesting, though.