Kratos has every excuse for looking like (and being) an absolute, muscle-bound monster. When the gods trick him into murdering his own wife and daughter, his only recourse is to stab, slash, and rip through the entire pantheon of Greek deities. To do that kind of thing, you need to bulk up a bit.
Luckily, Spartans are trained for combat from a young age, so Kratos starts with a decent layer of muscle. After that, it's all just fine-tuning. Chances are you weren’t brought up as a Spartan warrior, which is why you probably don’t have his extreme body. Never fear though, it’s never too late to give the lifestyle a try. Just give up all your worldly goods--yes, even the consoles (especially the consoles)--and do nothing but sit-ups, push-ups, and murderous rampages against the gods all day. Boom: insta-ripped.