Humana has more than 15% of the commercial inventory in downtown Louisville, KY, and maybe even still more than 15% of the entire Louisville metro area as a whole. I don't recall the exact numbers and searches aren't turning them up right now.
I'm not sure it's that unusual, statistically. If anything, one large company per city is maybe closer to the historic norm (especially if you count old fashioned company towns and more modern might as well be company campus towns like Redmond, WA) and something of the median or mode of city existence, at least in the US.