Muscular does not mean very heavy. Either Satoshi is so hilariously buffy he lacks neck, or he is fat.
Actually, yes, it does. Muscle is denser than fat, so the same amount of muscle makes you heavier than a given amount of fat.
However, in order to be
that heavy, you do need to be very muscular indeed, or else have a reasonable amount of fat. It is entirely possible to have that BMI and be fit (Rugby players do, for example, as do American Football players in certain positions, I suspect), but you certainly have to be putting significant effort into training to get that heavy without being fat, and people would definitely notice it.