I make 7oz size and they come out of the oven all puffy and beautiful and a few minutes later, shrink to half the size
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To a certain extent, that's the nature of a soufflé. They're meant to be baked and rushed to the table. When you see a photo of a soufflé in a recipe/cookbook, it was taken seconds after coming out of the oven.
Absolutely true - you can't get around the laws of physics (or actually. However, if your souffle did rise in the oven, even after it sinks it will still have the fluffy rich texture that is so beloved about soufflés.