No.  You *must* use very heavy wire and there's no getting around that.
I suppose if the cables were only 2' long then you might get by with
thinner wire, or you could accept having to sit there for an hour or two for
your battery to charge, but most people won't tolerate such compromise.
You can put it in a pretty bag, but you can't make it any smaller or
lighter.

Don't bother with the junk that connects through a cigarette lighter jack.
They're a waste of time.  On a warm day with a nearly fully charged battery
you might be lucky if you increase the charge within two hours enough to
matter.

Not really.  The one thing that makes cables good is that they have low
resistance.  Barring using silver wire or superconductors, they have to
be heavy to do so.

HOWEVER, the real question is why you would need jumper cables with a
rental car.  I could see needing them to help out another driver on the
road (which incidentally will invalidate most rental car contracts), but
if you're running into rental cars with battery problems you might want to
consider a different rental company....
--scott

I have a set of GOOD cables that fit in a round plastic flatpack.  The
cables are not
thin, but they are very flexible because of the high quality (many strands
of fine wire)
cable used.  They will fit in a very small space, and are easy to repack.

I dont remember where I bought them just now, but dont buy cheapo low
quality
battery cables.They will disappoint you.