I have seen what you're talking about with the onions but most of the time I've read it's best not to give them a haircut, so I never have.
Your zucchini there could be OP or it could be a hybrid. The OP version is called Round of Parma (similar to Round of Nice, which is a pale green color, lighter than yours) or could be the hybrid Eight Ball. Either way, I think they're best picked around baseball size and no larger. If you get a lot of rain, they can double in size overnight, as you said. It's not your imagination.
Enjoy your garden!