Your refund is not at all proportional to your income. It depends on your tax liability and the amount of tax withheld.
The tax withholding tables are designed so that you pay enough tax as you earn so that your refund or your tax due is as close to 0 as possible. In fact, if you have a large refund at tax time, this means that too much has been withheld from your paychecks during the year.