H1: If they are equal on other aspects of (physical & mental) ability, a bigger WR prospect will probably have a better career than a smaller WR prospect.
H2: NFL teams undervalue WR size, so a bigger WR prospect will probably have a better career than a smaller WR prospect drafted at the same spot.
H3: Bigger WR prospects are more likely to have exceptional NFL careers (they tend to have a higher ceiling than smaller WR prospects).
H4: Bigger WR prospects tend to have more fantasy value than smaller WR prospects, holding constant where they are drafted.
H5: Smaller WR prospects are more dependent on their QB than bigger WR prospects: the dropoff in production as QB quality declines is greater for smaller WRs than for bigger WRs.

I believe that these are all distinct – for any pair of hypotheses, you could imagine one being true and the other being false.

For all of these, “bigger” could be defined in terms of height, weight, or BMI.

It looks like this analysis focuses on H2 (expected NFL production, controlling for draft spot), with “bigger” based on height.