Mattress Support Board

We have a factory king size mattress. Would like to replace it, but my question is that the existing mattress overhangs the support board by about 3 inches on each side and and 4 inches at the bottom. I'm concerned that this board is not the right size for the king mattress and I should replace it, before replacing the mattress. Is this the norm with a king size mattress?

that's about how much I removed when I cut my king down to a queen.