why not 'can't' but 'couldn't'?

[joham]

I read this question somewhere before and have been puzzled about the original answer of 'couldn't':

---Have some more cake.
---Oh, I couldn't/ can't. Thank you.

I thought both answers are all right and I'm more likely to say 'I can't'.
Do both 'couldn't' and 'can't' sound all right in the reply? Could you explain why?
I was hoping a native English teacher could give me the answer. Thank you very much in advance.

Thank you.

[oregeezer]

Either is okay.
'couldn't' is usually taken to mean "I should not. It will make me fat!"
'can't' is more like "I am too full."