Like it or not this league is about size when it comes to the dominating forces at WR.
Larry Fitzgerald, Calvin Johnson and Andre Johnson are widely considered the best WRs today, perhaps the best of our time. The thing they share in common is they're all freaking huge. Demaryius Thomas is coming into stardom, another huge guy. Brandon Marshall, gargantuan. A.J. Green, 6'4" and 215 lbs. Marques Colston, monstrously sized. Julio Jones 6'3" and 220+ lbs. Dwayne Bowe over 6'2" and 220+ lbs. Vincent Jackson is a monster. Anquan Boldin is big. Jordy Nelson is huge at 6'3" and about 220 lbs. Dez Bryant is huge. Miles Austin is 6'2" and 220 lbs. Eric Decker is 6'3" and 215 lbs. And then you've got guys like Hakeem Nicks who aren't huge on listings but seem like it on the field. Then you've got the players that don't have it anymore but used to dominate, like Randy Moss and Terrell Owens...huge guys.
Heck even guys you wouldn't think of as big players, really do have decent size. Brian Hartline is 6'2". Roddy White is over 6'1" and was 207 lbs coming out, probably heavier now. Torrey Smith is over 6'1" and 205 lbs. Victor Cruz is 6'1" and 200+ lbs.
There are certainly exceptions, I'm not saying there aren't. I think Marqise Lee and Stedman Bailey are going to be great examples of exceptions. I just think you do have to keep that in mind when you're looking at the very top of the WRs class and you see guys like Keenan Allen (6'3" and 220 lbs) and Justin Hunter (6'4" and 200+ lbs with plenty of room to grow out more). Those guys are up there because they have the potential to be dominating forces at the receiver position.