I like to think that, when blowing up, the juice that turns you entirely blue has a property that allows you to stretch your body more that is humanely posible, thus adding more flexibility

That would explain why when Violet blows up, she doesnt show any pain, only concern, and why her clothes don't get affected by the new body size since it gets stained by the juice 🤓☝️

I hope this answer helps you out with your thoughs about turning into a blueberry being an extreme pain xD