Why can only 80% kimchi be filled in biology elective courses?

Kimchi is 80% full, kimchi water is not easy to overflow, and kimchi is more likely to succeed.

Fresh vegetables will produce moisture under the action of pickle brine. At first, kimchi fermentation will produce gas. If the pickle jar is too full, the pickle water will overflow.

However, in fact, because the volume of vegetables will decrease after they come out of the water, the gas produced by fermentation will vary from place to place, and not all kimchi will overflow. For example, submerged vegetables are different from floating vegetables.