I'd favour big dominant wrestler types like Islam or Khabib to beat a HW NFL/rugby guy most of the time but plenty of 155'ers would be vulnerable. I think 170 is the threshold where the fighters start getting big, strong and powerful enough as standard to be able to handle pretty much anyone.
Those NFL and rugby guys are extremely athletic in addition to being big and strong, its not like its just some random fatty or a bodybuilder who can barely move. They're also conditioned to take heavy impacts and physical punishment way beyond what some 155 or lower MMA guy can dish out. If you're used to having potentially multiple HW sized guys running into you at 20mph you're probably not gonna be phased by Sean O Malley punching and kicking you.
Btw i'm not one of these "A level athletes" guys at all. I actually think people downplay the quality of MMA athletes etc. But then people also often go too far in the other direction thinking that fighting is some kind of impossible rocket science and that simply being trained can make some 5'4 140lb dude practically invincible. Its not glamourous or nice to accept it but at the end of the day size, strength, power, athleticism etc are major factors in a fight.