@ Surebet - Does your girlfriend want WANT a vest or a chest rig type setup? Australian tactical nylon manufacturer SORD makes a really small chest rig called the MFF (military freefall) and it is really small.
Military Moron review here
Product page here
I'm 5'8", I use it for milsim paintball and I think its great for anyone my size and smaller. Coupled with anything you can hook up to the blast belt and drop leg rigs, there should be more than enough pouches - and weight - to deal with.