I think 240 is ideal for him. Pre-champion Jones walked around at 230 Lbs. add 10lbs of functional muscle, he'd be stronger, just as quick, still capable of going 5 rds while pushing a relentless pace. Jr is 6'4 243. So clearly that's an adequate size to be a champion at HW. At that HW, done properly I'd see my boy Jones taking the belt there too.
But you are forgetting that JDS has lightning quick hands with KO power for days, Jones hasn't really shown KO power like JDS. Maybe if Jones had more muscle he could have one shot KO power in HW division.