First of all, I was going by a picture I had seen of Arnold on vacation at the beach with his family. He was wearing a speedo, and all of his muscles had turned to flab. And the older you get, the harder it is to rebuild lost muscle tone.
Second, I'm not saying he hasn't returned to the gym since leaving office, but he wouldn't have to try and rebuild some semblance of his old physique had he maintained his exercise routine while he was in office, for fitness reasons if nothing else. Even if he had kept up with his exercise routine while in office, he wouldn't be in as good of shape as he was pre-office. Age catches up to everyone physically, no matter how physically fit they are. Just look at all the wrestlers who continued performing well past their prime like Hulk Hogan, Ric Flair, and Terry Funk. All look great for men their age, or men half their age, but none are in the shape they were in 30 years ago.
I'm just gonna have to wait and see I suppose.