Why is my self-employment tax so high?

I play music on the side occassionally and I received a 1099-MISC for $600 I was paid in 2016. I entered this into TurboTax after all my regular wages and other tax info. When I entered the $600, it dropped my refund by about $300! But in TT it says my self-employment tax is only $85. Why is this happening? Thx!