It's genetic mostly.
Hypertrophy is not only caused by high intensity and low reps. If you do any exercise long enough this will lead to hypertrophy to some extent (the extent is genetic and proportional to the stimulus you are applying).
Those skinny elite distance runners you see that run 100+ miles a week, that's genetic. Their body simply doesn't want to build muscle.
For most white people, if you start doing long distance running (such as marathon training), your legs will almost certainly get a bit bigger; you shed fat but you will build muscle.
I would also like to point out, that when highly trained distance runners suddenly stop running, and continue eating as normal (like before a big race), it's easy for them to put on 15-20lbs. How so? Glycogen. Highly trained runners constantly have their body in "low energy mode", and when they stop running, the body is like "shoot! Don't know when we'll next get this chance to store up on energy, better do it now!"
And as a result muscles start storing starch in the form of glycogen very effectively.
One more thing, sarcoplasmic hypertrophy is when the muscles don't grow bigger, but the muscle adapts to have more water in it (to help muscle get energy and respiration done more effectively), this is less common in distance runners. But again. genetics.
So overall it's mostly genetic (your body frame), but the type of exercise you do, and how often also plays a big role in hypertrophy (gain in muscle mass, fiber size).