No, if you are filing married separate, you won't see a larger refund due to your W-4 Single withholdings. That's because when you file separately you are subject to the same tax rate as a Single person. It's a good thing you were withheld at the single rate during the year or you might have owed quite a lot in your return.
You're choosing a filing status that is not at all beneficial to you, compared to married joint. Unless you have a specific reason for filing separate, you should consider the joint filing with a much lower tax rate.
