The battery packs for jumping cars (at least the ones I've seen) are generally more of a size appropriate for putting in a trunk than a backpack or pocket.
I had one a while ago when road tripping in the US, served me a few times, it won't fit in your average jeans pocket but it easily fits in a jacket, you could fit 20+ in your glovebox
https://cdn.mos.cms.futurecdn.net/rP7cR7ffBYrpfZ2Yz7FHdE.jpg
https://cdn.shopify.com/s/files/1/0096/6764/1402/products/AC...