I didn't even know what an "8-head" meant until I just Googled it. It's a joke off of the word "forehead" meaning that someone's forehead (4-head) is so big, it's twice the size as normal, so it's an 8-head. This actually made me chuckle...
In this case, JW was comparing the view of his forehead in the monitor to BK's forehead when he made the joke. It was self-deprecating because JW does have a large forehead and a receding hairline,and to be real, BK's own hairline does accentuate her forehead. In terms of put-downs, this was pretty tame.
Personally, I think BK is cute in a natural girl-next-door kind of way. Her forehead is fine as far as I'm concerned. I'm the kind of guy that looks at the whole package, and with BK, I see a successful world class athlete, millionaire, and somebody who seems like a nice person. If I were single and she was interested, I'd date her in a hot minute. The fact is that some people have long noses, some short, some people have a weird chin line... there are always imperfections we don't like about ourselves. It sounds like BK is sensitive about this topic, but if JW apologized, that should be the end of this tempest in a teacup. At the end of the day, BK has a few Grand Slam trophies and JW doesn't, so she wins!