Well, why is the controller asking the pilot?! The controller needs a copy of the 7110.65 right now!
However to answer your question, the B757 is considered a large aircraft but treated like a heavy, depending on where it's at in sequence.
By definition, the B757 is classified as a large aircraft since its max operating weight is under 255,000 pounds.
If the B757 is first in sequence (no aircraft in front of it), then the aircraft is treated like a "heavy" aircraft to the succeeding aircraft, regardless of type. This is due to the wake vorticies the B757 produces, which are similar to that of a heavy aircraft.
If the B757 is a succeeding aircraft (behind another aircraft) and no other aircraft succeeds it, it is considered a large aircraft and nothing special applies.
B757s are treated like heavies in many situations for reasons of wake turbulence. When it comes to holding for wake turbulence on takeoff, a pilot must hold for 2 minutes when departing the same runway or a parallel runway that is separated by less than 2,500 feet, or if they are operating on a runway with a displaced landing threshold and a departure follows a heavy/B757 arrival or an arrival follows a heavy/B757 departure. Pilots cannot deviate from the 2 minute rule, either.
It gets more confusing from there and a 3 minute rule applies when an aircraft is making a low or missed approach utilizing opposite direction takeoffs on the same runway or parallel runways separated by less than 2,500 feet. A pilot can deviate from the 3 minute rule, but not if the preceding aircraft is a heavy jet or B757.
Other than that, I don't know why the controller was asking the Delta if he was a large or a heavy unless the controller wasn't sure of the 757 was a -200 or -300 series. -300 series are a heavy aircraft as their max operating weight is over 255,000 pounds. Delta did acquire some 757-300 series aircraft from the Northwest merger.
FAAO 7110.65S CHG 1 3-9-6c