I've been using the small bottles for years now. Carrying on a full 16 oz. bottle of solution was not ever something I was willing to do.
I don't use straight saline; only the full soaking solution that does have preservatives. The small bottles I get from my eye doctor hold 2 oz., so I do go through that in around 30 days since I travel frequently. Naturally, I'm careful when refilling the bottles (carefully washed hands, no funnels, etc.), and I switch out the small bottles every couple of months. My doctor will give them to me any time, so I usually get 2-3 of them a couple of times a year. A large bottle of solution normally lasts me about 8 weeks, with about half of it going into the small bottle.