The Perkeo II is about the smallest 6x6 folder there is, at least amongst those listed on the 120folder.com site (not an exhaustive list, but a long one). It’s also among the lightest, weighing less than all except the Foitzik Foinix, Lomo LC-A 120, and Bessa 66. Actually, the Perkeo II weighs less than any of the 6x4.5 folders listed. Its Synchro-Compur has a wider range of shutter speeds than those three, and some other features I like a lot (frame counter and lock so you only need to use the red window for the first frame, double-exposure lock). It fits in a pocket and I carry it in a small chest pack under my parka when I’m skiing.
Like DWThomas, I also use an Ercona II. It’s significantly bigger and heavier so doesn't come close to being the lightest-weight MF folder. But with the 6x6 insert it's a 6x6 with a 105mm lens, which is sometimes a nice thing, and doesn’t use film any faster than the Perkeo. The challenge is finding the insert.
Having used both, I prefer the size of the Perkeo for skiing, but carry the Ercona when walking around. The Perkeo is truly small.