I'm sorry to hear that.

[Xinfu]

situation:

Peter has returned to his room after going to a party, and John hears Peter say:

-I had the smallest helping I've ever had. (Is it natural English?)

Then what should John say? Can he say this?:

-I'm sorry to hear that./You have my deepest sympathies.

[Rusty]

All natural English.
The last sentence, although proper, is too high a register for friends to use, so it would be taken as sarcasm.

[Xinfu]

Thank you~