A lot of really strong guys don't necessarily have a lot of muscle definition. I'm sure Khabib is naturally strong as hell, but there's also a difference between gym strength and functional strength. He may or may not be a beast on the weights in the gym, but he undoubtedly has freakish grappling strength. It's probably a combination of having a really strong core, really good balance, and excellent grappling technique. When you combine all of those things, then yeah, I'm sure it probably does feel like you're grappling with someone who is significantly bigger than you.
I wrestled around with a particular guy my freshman year of college once. Once. I probably outweighed him by 40 pounds. He absolutely ragdolled my ass. It felt like he had 100 pounds on me. Turned out he was a state champion wrestler in high school and was just over it and didn't want to continue at the college level.