The jumper cables I had were not capable of carrying full starting current from my battery to his. That's why they are called "booster cables" rather than jumpers (on the packaging).
You want to have big thick jumpers. The thin little weenie cables that often come in commercial roadside emergency kits just don't carry the current efficiently.
Heavy gauge cables are more expensive, but work much more effectively. I had a good illustration of that just the other evening. As I was warming my car up to leave work, a fellow asked if I could help jump his car. Temperatures were around 0 F (-18 C). When I drove over to where he was, another guy already had his thin jumpers on and they were trying to start the car. No luck. We decided to try my (heavy duty) jumpers, and finally, after some effort, got the car started.