I apologize Feng Xu for the late response, I am not a fan of medium notifications about comments.

The short explanation would be that const SizedBox() is kind of compile time constant. No matter how many times you use that / create, no additional objects will be created.

The most dev friendly explanation that I was able to find is https://stackoverflow.com/questions/21744677/how-does-the-const-constructor-actually-work

Hope that clears up the concept a bit