I'm a jersey stuffer I guess. Personal pet peeve - swaying weight on the bike when I am mashing. It makes the bike feel unstable. I try to pack as minimally as I can so the jersey constraint is a good benchmark.
I carry a small Cooks Bros multi-tool, CO2 and one lever. The rest is food and cell phone.
Crap jammed in your jersey pocket sways more than a small, tight bag on your saddle (I know, I ride both ways). But if you really want something smal for the jerseyl, the Jethro Thule will do the job and is lot lighter than a cell phone (and smaller too). Just using that for comparison purposes.
I've got a small stubby 15mm from McMaster.com that is lighter than a Craftsman stubby but slightly longer than the Jethro. When I'm going ultra light I use this
mini tool which should let you shave some more weight.