True that the ultimate in non bulky is the inflatable. This is a Type 5 PFD. If you go that route, you really should wear it in front..most people wear it in back. If it's on your back (turned around), it's hard to use if you are in the water, distressed and in need of it. Make sure you try it on dry land first so you know how to use it and make sure the C02 is engaged.
For a less bulky PFD, and a Type 3, try the PFDs that kayakers use. They need lots of room as they need to move their arms. A favorite, of mine, are the Astral PFDs. Harmony has a decent PFD as well. These have pockets for fuel, cell phone, etc...and keep you warm but are less bulky.
The current PFDs (belts with an inflatable flotation device & CO2 canister) are anything but bulky. You can find them at almost any surf shop. In NY, people are beginning to get checked by the Marine Patrol to see if they are carrying PFDs when they take their SUPs offshore or out into the boat channels. I think its wise to carry one; I do.