Because he is a teleevangelist who is full of rubbish/shit, if you get rid of the rubbish there is so little left that the rest could fit into a matchbox sized coffin.
It is a cruel comment but one that is characteristic of a typical genre of British wit: this does not mitigate the force of the comment but puts it into its literary context. Compare Churchill's comment to Bessie Braddock:-
Braddock: "Winston, you are drunk, and what's more you are disgustingly drunk."
Churchill: "Bessie, my dear, you are ugly, and what's more, you are disgustingly ugly. But tomorrow I shall be sober and you will still be disgustingly ugly."