As NFL defensive backs get taller and taller in order to combat guys like Dez, Calvin Johnson, D Thomas, AJ Green, Julio Jones, etc., the shorter WR's will have success against them.
Guys who are 6'3" tall simply cannot change directions like a guy who is 5'10". The tall corners can run down the sideline with a guy like Dez and have a chance to keep him from making the catch. That is why they are necessary for today's defenses. However, guys like Cole Beasley and Randle Cobb can make cuts that the tall guys simply can't make. Those guys win with separation and speed instead of catch radius, height and jumping ability.
The big DB's are having some trouble with Beasley, Cobb, etc.
However, Cole has Dez and Cobb has Nelson to take the double teams. If those smaller guys were the #1 WR's for their team and they were getting the double coverage and were getting bump and run with a safety over the top, they would be far less effective.
So, I think that the smaller guys are having success because the taller DB's can't keep up with them and because they're flying under the radar a little bit in the shadow of true #1 WR's.
Antonio Brown is a little bit of an anomaly in the NFL. He is pretty special in a lot of ways and guys like Beasley, Cobb, and T.Y. Hilton don't necessarily belong in the same category as Brown.
It is all cyclical anyway. In the 70's, the CB's were like Mel Blount size wise. Bigger guys who could mix it up with the WR's. Then, they changed the rules to keep DB's hands off WR's and in the 80's, a lot of teams started getting receivers like the Smurfs in Washington and the Claytons in Miami with Marino. Defensive backs had to be shorter and quicker guys like Darryl Green in order to cover those types of WR's. Most NFL teams had CB's in the 5'9" - 5'10" range and they were fine for the most part. Then in the 90's, along came Michael Irvin, Hermin Moore, etc. and those shorter guys were having real trouble with them. I still remember Darryl Green, the perennial All-Pro having fits with Michael Irvin. Green would have good coverage on Irvin but Michael would just out muscle him for position and catch the ball anyway.
That was somewhat of a turning point as team started moving more towards larger CB's to cover them. By the 2010's, everyone was trying to find the larger CB's to combat the Megatrons of the world. So now, the smaller, nifty WR's are again finding room and success with their quickness and speed, and the teams with all large corners are finding them more and more difficult to defend against. The rules now favor the receivers more than they ever have and small guys will continue to be effective.