Hi Herbinator, I am a pedorthist, trained in analysing the biomechanics of the foot. I am not seeing yours but I agree with OWC that it might have to do with your arches collapsing. I'd also rule out medical reasons first.
Overpronating is a big thing for sudden size increase and also for bunion appearance. Bunions appear because when you pronate too soon and for too long on your gait, you toe off at the wrong spot and you end up putting lateral pressure on your first metatarsal joint (great toe). While the joint is pushed out, the ligaments hold your great toe in, and then there it is. The bunion develops more and more with pronation, cartilage gets worn out and smal growths of the bone may occur.
As of sizing, I don't know what shoes you like or are good for you, but I know Brooks works with split sizes because I work with them. So you can buy one shoe each size to accomodate correctly your arches and feet in general. I don't know if all running stores offer you this option. If not, look for a Foot Solutions near you. You can get a complimentary foot/gait analysis and get the split size through them. Just my 2 cents. Good luck.

one thing I did not mention and just would like to add. There is so much a shoe can do in terms of preventing pronation. If you have high arches, you might still have a gap between the shoe arch and your arch, which still allows your arches to drop. High arches and bunion can be very efficiently taken care of with a type of orthotics called CBAS. It is the highest technology in orthotics you can find in the market. Very different from the pancake flat very common to find that lets you throw a party between the foot arch and the insert. CBAS have reversed certain cases of bunions. If not reversing, they will for sure keep it from developing. Not all doctors are interested in giving you the tip on how to avoid the bunion. Surgery is more lucrative. What they don't tell you is that even with surgery, the bunion tend to come back if the gait is not corrected. You need a very caring and sensitive podiatrist to tell you that. I'll be happy to give you more information within my limitted knowledge... I hang out at the Running Moms thread daily if you need to find me around.