Because it came up (not on this site) when the DK Metcalf 100m run happened, the 100m is probably the race where bulk has the least negative effect on time. I would probably agree that DK is a little too top heavy, but his size shouldn't be much of an issue.
Ryan Bailey has a PR of 9.88 @ 6'4", 225 lbs.
But I do agree with your other points about nutrition, training, and equipment. It was part of the reason I stated that Hayes ran his PR in 1964. I think the track conditions were less than ideal (especially with today's standards).