This is how they are describing the Lesnar/Undertaker match at Summerslam. However, it just makes no sense.
It's too big for Wrestlemania. But apparently, it isn't too big for Summerslam. Even though Wrestlemania is not only just bigger than Summerslam but actually WWEs biggest event all together.
I think it's just another one of these things that WWE says to try it make it sound like it has some kind of meaning and importance when in fact it doesn't mean anything.