Swollen spark plug wires

[Horsman]

Anyone ever seen anything like this; see pic?

Ordered the Petronix custom fit plug wires and here is what I get after 250 miles. Most of the wires are starting to do this close to the spark plug end.

I am running the factory distributor with a Crane electronic ignition conversion on a 402. Wanted to keep engine compartment stock appearing and hate making plug wires...any ideas of the cause or wire recommendations appreciated.

Thanks in advance.