You literally have no idea what you're talking about. Cormier IS fat compared to other fighters. It's not just how his fat is distributed, his body fat % is high for a fighter.
Just because you have good work ethic doesn't mean you will have a low body-fat body. Cain works extremely hard, but how's his diet? Does he even WANT to have low body fat %? I doubt it. Having some fat helps you cardio-wise. Just because you're more shredded does NOT mean you have better cardio or that you train harder.
Genetic plays a big factor, but EVERYONE can become shredded. Some people just look better than others while shredded, but everyone can get to that point except ones with rare diseases and such.
If you train right and have a good diet, you will become shredded. Bad genetics won't stop that, just delay it a bit.
The only thing I agree with you is that physique doesn't mean much when it comes to fighting. Cormier and Cain are top three HW and they carry around a lot of fat.