It's not really "muscle memory". He used PLENTY of drugs, ate alot of good food, and lifted weights very heavy for years. The drugs (high dose AAS and HGH) caused hyperplasia in addition to hypertrophy. Most naturals only get muscle gains from hypertrophy. Therefore Kevin has alot more muscle cells than most people (they are just smaller now), so when he starts training and goes on cycle all those cells just get bigger. Since he has more muscle cells he grows bigger and quicker than most everyone else. Ronnie and Jay could do the exact same thing it they wanted too. That why anyone that has ever used high dose AAS has a big advantage over a lifetime natural, even if the guy has been clean for 3 years or more.
It's kind of like a really obese 500lb guy, if he loses weight and gets down to 180 he had better be very careful because he can add on the fat 100 times faster than anyone else.
In other words Kevin's rapid response to weight training is more to due with years of drug use than him being a "genetic freak".