There have been several long threads on here about larger seat bags and seat post racks. I recently purchased and used a
Carousel Design Works large seat bag. This thing is a total work of art. The craftsman ship is excellent and Jeff is a really cool guy to work with. He also built me a custom frame pack for my Niner. The large seat bag holds a ridiculous amount of gear, does not hit the legs when riding, and is unnoticeable when riding. I got a 35 degree down sleeping bag, synthetic fill vest, thermal bottoms, gloves, and balaclava inside the pack and still had room to spare.
For those looking for a large seat bag or seatpost rack to hold a lot of gear I highly recommend looking at Jeff's stuff. He builds all the bags himself in the Sierra Foothills. Give yourself some lead time as his business is picking up and he mentioned he is working on several development projects (sounds like cool stuff). He makes smaller sizes as well.
For full disclosure, I am in no way affiliated with Jeff, Carousel Design Works, the bicycle industry, or any bike shop. I'm a desk jockey, IT, geographer who loves to ride bikes.
Here is a pic of my loaded Niner from this past weekends bikepacking trip in the Angeles National Forest just north of LA.
Bathroom scale weight is 42 pounds. That includes bike, gear, and loaded Wingnut. Does not include water or fuel.