First of all, no you are far from petty. It seems like your kindness and background has been taken for granted. Even though you are well to do, they should be more understanding that you also have things to pay for (daily expenses, mortgage etc). Moreover, I believe you will be more willing to pay for them if they are the ones who fight to foot the bill. But anyhoo in this case, your gf is the problem and not her parents. She should be the one telling her parents about this, and if she doesn't see a problem, then that is a problem.