Went up to HW, cut +20lbs of muscle to get back down. Hence his punch resistants and reflexes went to hell. Unlike FMW, if he never goes up to HW, he does take an L at some point b/c he was a risk taker but he slaughters Tarver, Glenn Jones and Calzaghe before that happens.
You nailed it. The move back down to light heavyweight was stupid. Plus, it was way too fast. Roy was so fucking stubborn that he priced himself out of the Holyfield fight (which James Toney took and destroyed Holyfield) and then cut 20lbs of muscle to fight Tarver for much less money. It was all mapped out for Roy at heavyweight. Holyfield, Tyson and possibly Lewis to really cash out. The only one of those fights he'd have lost was to Lennox.
That said, he is the best fighter I've ever seen. At his absolute best, I'd favor Roy over any fighter under 200lbs in history. Prime cruiserweight Holyfield and Ezzard Charles would have been pick em.