When I put in a $17,480 charitable donation, I only receive a $593 credit. What is going on? Shouldn't it be closer to $4,370? A $36 donation receives an $8 credit.

My charitable donations are not receiving the credit they should.  I can put in a small amount like $39, and have an $8 credit, super, that works ( I thought it's usually .25 savings for every $1, but $8 is close enough.).   But what is bothering me is when I put in a $17,480 donation, I only receive a $593 credit.  What is going on?  Shouldn't it be closer to $4,370 if I go with the similar scenario?  I hope you can help.