If i transfer common income to the higher earner between my wife and i the combined refund increases, why is this?

[TurboTax-Esther]

Does  your spousal credit increase? ie is her income below $11,635?

Provided you are in the 15% tax bracket, by adding investment income of the spouse to your income, can increase the spousal credit you can claim thus reducing taxes payable. (her income is lower so your credit for her is higher)