I'm not so sure BMI is such a big deal for WRs. It may have more importance for RBs. Hell, AJ Green looks skinny compared to a lot of WRs.
It might be less important. There's a wider range of successful body types at RB than WR, spanning all the way from Charles/CJ2K/Spiller in the 27.X range to MJD/Turner/Stewart in the 33.X+ range. At WR you have Green/Wayne in the 26.X range and Andre Johnson in the 29.X range, but virtually nobody successful outside that territory. DeSean Jackson is the one guy below 26 who has had some sustained success, so I guess you could count him. He does underscore the idea that if you're that slender then you have to have insane speed/quickness to compensate, which is also true at RB.
Anyway, BMI provides a frame of reference that can help you understand a player's physique, playing style, and skill set. For example, White and Parker both checking in between 26.0-27.0 suggests that they're going to need to thrive on finesse/speed/quickness because their frames are relatively light compared with a typical big possession guy. For example, Fitz/VJax/Dez are all substantially heavier relative to their heights. Ditto Benjamin and Evans from last year's draft.
Given that White seems to lack great speed and plays more like a physical possession type of receiver, you would've liked to have seen a height/weight shaded more towards the 28+ BMI range. That's where Hopkins, Watkins, Evans, and Benjamin all landed in previous years. Patterson too IIRC. So while 6'3" 215 might seem like a "big" receiver on paper, it's a little bit on the lean side. That's not a death sentence or a guarantee that White will bust. Just another piece of information to factor into your analysis and help you understand what this player offers. My beef with most mainstream and casual pundits is that they tend to look at height or weight in isolation. So 6'2"+ is always "good size" for a WR regardless of bulk and 215+ is always "good size" for a RB regardless of height. It's a little more complicated than that. For example,
Melvin Gordon's 215 pounds on a 6'0 5/8" frame means something very different from Michael Dyer's 218 pounds on a 5'8 1/4" frame. To a lazy analyst, they have similar size. In reality, their body types are radically different.
It's a useful thing to look at, especially in conjunction with 40 times. For example, k
nowing that Langford/Coleman/Gordon are all relatively lean for a RB, I would definitely feel better about their prospects if they clocked a fast 40 time. It's less important for someone like Cobb, Ajayi, or Dyer because those guys have the bulk and power to function effectively in the NFL even with just 4.5 speed.