That's a horrible comparison whoever said that, if for no other reason he's going to measure out at 6'1"-6'2", 205 lbs compared to Beasley's 5'8" frame.
I thought it was an odd comparison at first considering that Kupp is 6-2 and Beasley is 5-8; however, he does play in the slot primarily and he gets open more by being a precise route runner and having good scheme knowledge than by being physically imposing. He does not have the Beasley quickness and he appears somewhat high-cut (long legs, short torso). My comparison would be if Jason Witten (the old version) was smaller and played WR (i.e. He is more crafty than athletic).