Being Linemen, they are indeed required to line up on the Line of Scrimmage before the Snap, unlike the others.
Because this places them nose to nose with the opposing CornerBack ~ whereas the SlotBacks and Flankers are allowed to align a couple steps back, thus affording them more room to maneuver ~ they need to be much bigger.
*Please do note, though: Schematically, the Split End is whoever lines up as the Split End...But for the purposes of these Scouting Reports, I am applying this NomenClature to those who are Split Ends by trade, so to speak: Those whose Skills Sets apply best to that particular Job Description. What their Coaches might do with them, once they're drafted, is another matter all together. Ditto, the Flankers and SlotBacks. In any case, many play multiple Roles.
The most compelling Attributes of a Split End are, in my view, and in no particular order:
Release ~ How Adeptly he employs Burst, Core Strength, Fluidity, & Combat to Dust Off the Defender
Routing ~ How Adeptly ~ Crisply, Intelligently, and Reliably ~ he Runs the Route Tree
Separation ~ Fluidity & Ricochet ~ Explosiveness out'f'is Breaks
Catching ~ Reliability & Technique
Navigation ~ How well he Navigates the Field in Pursuit of Yards After Catch
Blocking ~ Core Strength, Drive Power, Fluidity, & Combat
Routing ~ Masterful. A dedicated Craftsman within whom Discipline & Precision meet Fluidity & explosive Ricochet.
Separation ~ Exceptional. Very advanced with the subtleties of the Position, and remarkably Strong & Athletic.
Catching ~ Fair to Middling.
Navigation ~ Absolutely Outstanding. A Powerful, astonishingly quick, and dangerous Runner After The Catch.
Blocking ~ Raw but he gives good Effort, he's steadily improving, and this will likely quickly become a Strength.
His Long Speed is pedestrian, but the stuff that wins Games & Championships is the Short Speed and Agility, and Keenan Allen commands phenomenal Fluidity, Lateral Agility, and stunningly quick Ricochet for such a big man.
That combination, along with his extraordinary Routing, should make him an enormous Weapon at the Next Level.
Best of all, though, considering the notorious lengthy Incubation Period of most "Wide Receivers"?
He's ready to roll today.
Consensus Market Value