Being 20 Stone can't be healthy in the long term, whether it's muscle or fat; or whether it starts off as muscle and then 'relaxes' later in life.
I can just imagine there's a lot of long term extra stress on joints and back 'n' stuff; and maybe extra pressure on organs.
I spent a few days 'working' with a civilian body guard in Bosnia and he was a runner up for Mr UK 1990 (or early 90's), and he also auditioned for the 'new' TV programme Gladiators. He told me he wrecked his back by jumping over a six foot wall for the TV programme; it was the landing that did it. Two of his spinal column vertebra crushed the cartilage(?) in-between and he was bed ridden for a number of months. It was only a six foot wall; something any normal fit person should be to get over without a problem.
The guy - though obviously strong and fit - seemed to be almost 'disabled' to me.