Jay still has an absolutely massively developed upper body even without his legs.
Phil just doesn't have a lot of mass anywhere other than arms and maybe delts. He is different than most BB'rs in this regard, as few are considered among the best in the world with any outstanding muscular size throughout their bodies.
Even Dex, who you mention, packs a lot of mass all over for someone of his height. He is 3 inches shorter than Phil, yet he weighs more than Heasth onstage. He is not small by any means. He definitely has more overall size than Phil throughout his torso, no doubt about it.