http://www.aqua-calc.com/calculate/volume-sphere
for anyone who likes to get sizes and weights right, here's a fun toy that calculates the volume of any size sphere, and then you can use the volume to weight/ food calculator button to see the weight when it's filled with either regular substances or even thousands of kinds of food. yes, blueberries are one of them. XD see how much a blueberry of any size would weigh!
A brilliant link, and even more - "Heavy Inflations".
You pinpointed the main idea correctly. Just two words - yet this is the key. Thank you!
It's a sort of world-design consideration, whether the world allows heavy inflations.
The way I imagine it - human tissues are the same in imaginary world. So when a character becomes nothing more but a pool toy with all insides temporarily gone, her bubble will require a very specific and hard training to handle the large pressures liquids give. Will she want to spend several experience levels just to train being a blueberry, being pressed and milk-pumped? Again, and again, for months or may be years, doing it with gases in her imaginary world - until she is ready for juice?
Another way to imagine it - is letting heavy inflations happen. I'm for diversity - the more different fantasies, the better. This is an entirely different world, with different answers to everything, starting from "What is inflation?" If inflated lady is essentially a berry - then her skin is not just a membrane, it's a thick extremely strong rind. Then everyone can be a blueberry, and even when squashed they stay intact - if they can handle pressure differential the juice gives them, how bad could it be if someone adds a bit more? Liquids are almost incompressible anyway. ^_^
Great ressources for inflation writers if they get crazy technobabble in an inflation lab, test subject inflation, robot inflator or an advanced alien setting ;)
Visit the doctor's office: http://doctorbo.deviantart.com/