Why position of `[0]byte` in the struct matters?

[0]byte in golang should not take any memory space. But these two structs have different sizes.

type bar2 struct {
    A int
    _ [0]byte
}

type bar3 struct {
    _ [0]byte
    A int   
}

So why the position of [0]byte matters here?

By the way, I use unsafe.Sizeof() method to check the struct size. See the full example .