Reason 1:

That looks like more than a pushup bra. They look a bit too tight to be natural after 5 kids.
Reason 2: I don't have the stream link handy, but it was only a month or so ago during a stream that Nick went on a rant about how implants aren't "fake boobs" because the boobs are still real, just enhanced or some shit like that. He was really trying hard to convince everyone about it. It was a weird rant to go on.
Reason 3: Nick has really been showing everyone lately that he's a sexual degenerate. He has a St Andrews Cross FFS. I have no doubt in my mind he'd be into implants that oversized for her frame.