You're not wrong but there's absolutely no advantage to using more bac water. You'll use more bac water and get the same results. Plus, you'll have to pin more fluid than you would if you mixed 5000iu with 2mL.
I wouldn't do it unless you're injecting IM instead of sub and you'll be using 3mL syringes. It will be easier to measure out on a larger syringe if you used a lot of bac water.