Jump starting Prime diff from other Prius?

Voltage drop depends on the wire length and the wire gauge. By reducing the length, they can also reduce the gauge with no adverse effect on the voltage drop.

As the little jump pack is small enough to bring right to the battery, the cables don't have to be much longer than you need to reach both posts (or one post and some nearby metal). They can make little one-foot cables out of 10 gauge copper and have them perform equivalently to six-foot cables made of 2 gauge. In pounds of copper per foot, 2 gauge is also around six times as much as 10 gauge. So making the cables six feet instead of one foot would have 36 times the cost in copper. It's quadratic.

As you suggest, they could be just a little longer, say 2.5 feet instead of 1 foot or so. But that would still be 2.5², or more than six times the cost in copper.

 

Their bean counters might be more amenable to just increasing length/gauge of ground cable.

 

It can still happen that way. I used mine to start the neighbor's mammoth RAM 2500 a couple months ago. Itty bitty battery, great big truck, even bigger eyes. It probably spun that engine better than a new OEM battery.