I have carried jumper cables for many years on our trips but thought a jump started would have more uses.
As henzilla pointed out jumper cables don't work if you don't have another bike or car to jump from.
I ordered a XP-3 from Anti-Gravity and it arrived today.
I was surprised to see it's bulkier than my cables.
I might have been better off with the smaller Sport. I thought the 2 USB outlets on the XP-3 would be handy and the Sport only had one.

Here's a few pictures with my jumper cables so you can see the size.
The clamps and connector are the bulky part of the package.

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I could get a clampless harness for both of our bikes to reduce the bulk, but this would not work if I need to help another rider.