Even if you've got nimble salad fingers like I do, picking midges, small nymphs and tiny dries out of slit foam fly boxes or the like is an exercise in mounting frustration. Putting them back is worse and you know you're not taking the time, when streamside, to do it gracefully. A messy, disorganized fly box is the result. And, if you're like me, you live with that mess for most of the season instead of tidying it up when you get home.
Over the last couple of years, I've moved away from larger fly boxes and towards smaller boxes in an effort to increase my on-the-stream minimalism as well as the potential for horrific fly box messes. I almost always carry my camera and lenses, in my beloved Patagonia Stormfront Backpack, when I'm on the water. This means a no go for most vests and packs (although some hip packs will work). That said, I'm working only with wader pockets and -- when warm weather hits -- only with shirt pockets. Cutting down on big bulky items is a must. Enter small fly boxes.