You will need to review the income to withholding ratios but my educated guess is you both went from Single and1 or zero allowances on your W-4 to both Married and 1 or 2 allowances AND you both have a decent income. If you did not complete the W-4 worksheets correctly you would have had less withheld than desired thus your refund is less.
Another factor is if you have kids ... the combined income could have been too much to get some credits.
