It is not exactly extra information but the value stored in the variable does change.
The left and right hand side of the equals sign does different things here.
When you use the var keyword, it creates the size variable. Without this you cannot use the size variable. So it is needed.
Remember that the equals sign in programming is not like equals in math, it should be read, "gets the value"
So, the line of code in total does two things:
We create the variable size and it gets the value 30.
You are correct that the original value 30 get overwritten on the next line of code so it ends up not mattering what that value is at the beginning to determine what the value is at the end.