I would say both age and political relevance. Reagan looked like a million bucks even at 70. Fred is, well, a lot of it's the weight. He looks his age. I don't know if the fact that he's been out of office for so long helps or hurts him. I suppose you could argue either way. It could be a positive if voters associate the '90s GOP with common sense conservatism, fiscal sanity, limited government, economic growth, etc. But voters may also look at Thompson as a 9/10/01 candidate who hasn't really done any heavy lifting in this GWOT world. That could go either way I suppose.
Thompson looks pretty good to me (and I like lookers): mature and kind of sexy.
A few extra pounds on a big guy is not a liability. (Look at pudgeface Bubba)
As long as he's healthy mentally (in a conservative way)and physically and picks a good VP like Hunter...I'm fine with it.
So it shall be!