I don't think you can really compare with your "expat friends in other professions". It sounds like you are employed on "local terms", so don't expect to be paid the same amount or have the same amount of leave as someone who was sent over here by their company on an "expat package". You need to compare like with like - how long have your colleagues with big cars been working at the firm? Are they more qualified/experienced than you? (And can they do a wider range of work because they speak Cantonese/Mandarin?)
But in the end, the only way to find out if you are underpaid is to find out what other people in comparable jobs are getting paid.