I think the fact the Jon Jones didn't have his normal size and length advantage over OPS was also a big factor in his performance. Jon's greatest weapon is his reach and length and his ability to stay at a range where he can hit you but you can not hit him. He couldn't stay at that range vs OSP and he couldn't stay at that range vs Gus. He is still so talented that he just can win at 205 vs anyone regardless of their size but I am very curious to see how he would do at HW dealing with guys like JDS or Reem or Werdum. 
Combo of both and i also think he could have been unsure of how his cardio would hold up with all the muscle he put on. Who knows, only he does
you could see it was not how he usually performs, but it didn't look like OSP had much to do with it.
Good point op, the only thing is at hw he might have a speed advantage, although his lack of punching power may be a problem.
It's all interesting, the whole bulking up thing. Cormier said he was surprised by how strong Jones was in their first fight, and he's a guy that has been going at big strong guys forever. 
Both
The obvious answer is both. Why is everything so black and white with you guys?
OSP's Ring
He completely dominated the fight. He fought the #6 guy in the world and was only punched twice in 25 minutes. He won a unanimous decision and one judge had it 50-44.
Also he said he was pretty distracted coming into the fight. Not only had he not fought in 15 months, his mother had her leg amputated the week of the fight and is losing her battle to diabetes. Also I'd think you have to factor in he was in jail in the middle of camp, just weeks out.
People are overanalyzing this.
It was an easy fight where Jones wasn't pushed or threatened in any way. Both guys looked perfectly content to have end in a 50-45 decision.
I think his inability to hurt osp in anyway cept to kick his knee is a concern.
Neither
OSPs unwillingness to engage
Size
lots of column a, little of column b