Why do some christians joke about Buddha being fat but when you joke about Jesus they get angry?

I have some christian family and friends and they never have a problem if someone jokes about other peoples religion, in fact they join in and take the piss out of them, but as soon as someone makes a joke about Jesus or christianity they get angry, why is that?
Update: I know the laughing Buddha is not Siddhartha Gautama, but it would have taken to much space to explain.
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