Could be 20 of muscle, 5 of fat. Often when people say "of muscle" it's just a casual way of saying "and most of it is muscle".
Even if true, it remains to be seen whether it actually helps in any way. Bulking up is great for power hitters. Not so much for pitchers. Bench press work is known to reduce throwing velocity. Pitchers getting into shape usually involves stretching, mobility and flexibility work, core and lower body strength, balance and repetition, learning new pitches, and sports psychology. If he put that much effort into simply becoming a sexy beast then I have to wonder how much time he spent, you know, actually becoming a better pitcher.