I haven't had opportunity to test this, but I submit it to the collective for your consideration. Maybe women here that wonder what really big breasts would be like for them could try this method to experiment with sizes.
And maybe someone with more math and science could work out some sample fills, with amounts of water and weights and all that, to aid experimenters.
You can find 36-inch balloons online for about $5 a pop (hehe).
Smaller ones (12" etc.) are usually at the corner store and/or easily obtained.
Put a smaller balloon inside a 36-incher, inflate it about half or 3/4 full, and tie it off.
Then add water to the big balloon to desired size. You can tie it off, but it's re-usable if you just twist the neck and tape it down into place.
Depending on how much risk you want to run (of the big balloon breaking), you could go quite large -- but going to 12" diameter or even 18" the large balloon is pretty darn stable.