Past:
Wendy Bellisiamo umbrella stroller fits folded but the front wheels rubs just a little.
Joovy Caboose Ultralight sit and stand fits folded when resting on the side but top wheel rubs on the headrest if it is down.
Current:
Some European tri-fold (cant remember name) fits with room to spare.
I saw another tri-fold that was ~$500 but can fold into the size of a small carry-on bag that can fit into the overhead bin of a mid size airplane (obviously not the puddle jumpers) it was pretty cool. Ive seen Euro/Asian umbrella stroller that are much more compact but not readily available here. I don't think it is the babies, rather we like massive understorage and long/tall handles to fit the paretns. If you go over seas, checkem out! We make the kids walk as much as possible so they were never really used anyhow.