Bigger, faster receivers have more opportunities to receive over smaller, slower defenders.
Drafting is inherently an exercise in probability judgments, 40 times go a long way in informing those probability calculations to the extent that they inform drafters about how fast the individual can get from A to B. Generally, this goes along way towards informing potential drafters because of its foundational suggestion-someone that cant be covered has a greater chance to receive, GENERALLY.
But the physicality consideration is only baseline. It allows extrapolations about potential but there are many factors that can ultimately negate both realization and application of that potential.
Ultimately, us on the outside dont have access to the facts that these decisions turn on. And how those "facts" are internalized isn;t necessarily science either.