Should adding a 1099-G for disability income from the DC government cause an increase in my DC state refund?

Adding income does not necessarily increase your refund.  Was tax withheld from that income?

Yes income was withheld (about 10% of it). However, the increase to my refund is 5x how much was withheld.

The refund should not have gone up so much. That being said, refund could have gone up a lot more due to other credits, such as EIC, etc. I would double check the tax withheld entries for DC as well. A transposed figure can also create an excessive refund.