As has been stated over and over again you can control how much weight a kid puts on. I don't why everyone just assumes all guys are going to add 30-40 pounds of weight. Derrick Allen could easily hit 230 if you wanted him too but Balis plan for him is to keep his weight at 215. If they value McDuffie's quickness off the edge they can easily keep his weight at around 255-260. Hell Anthony Chickillo was built up to 280 pounds to play 3-4 DE at Miami. When the Steelers drafted him they remade his body and got him down to 260 to play 3-4 OLB. BTW he had two sacks Sunday playing 3-4 OLB.
Allen's a bad example, doesn't need to ad size to play his position.
Anthony Chickillo is a better one. Chickillo was a 6-3, 225 lb player in HS not a 6-6, 240 lb like McDuffie. To get Chickillo strong enough to play line in college, he put on a lot of weight to get there almost to 280 lb as you mentioned. The Steelers saw something else and took a mature, 6-3, 267 lb SDE and went to work on him but he already had 4 years in a major S&C program and it's year 3 of his pro career.
To get McDuffie up to college strength, he'll have to add mass and his 6-6 frame dictates there will be more mass needed to cover that frame. He's also shown a propensity to put weight on and as his larger frame is filled in, he's going to add more weight than a 6-3 player. It's hard keepin weight off some kids. Who would have thought Okundeji would ever be a 260 lb DE? He was a sub-220 pounder during the camp circuit before his Sr. year. He's closing in on 260.
As far as WDE vs SDE, that gets over blown. To me, his film shows a good athlete but not the burst the level of WDE ND should be targeting... Joseph Ossai, Azeez Ojulari, Jarell Cherry or current OLB - Adam Anderson. Those kinda guys. His film and size look a lot closer to Devin O'Rourke than the guys mentioned above.