Genes is the most important thing when it comes to body type/size etc etc.
I've had relatives who REALLY wanted to gain weight, they tried eat as much as possible and they worked out, but they never gained a lot of weight.
I've got a good build for strength, wide everything (hips, shoulders, knee caps, ribs, etc etc). I also have tree trunks for legs. My body is shaped like a broad rectange.
The best adjective to desribe my body would be 'blocky'.
I took after one side of my family, my brother, took after the other.
He's very V-shaped, and very lean & narrow, he has chicken legs, and a 'bird' (flat) chest.
We have completly different body builds.
My thighs are literally twice the size of his.
But its all pros and cons.
I'm stonger, he has better stamina.
I will most likely be overweight later in life, and he most likely won't have to worry about his weight.
We've always been the same speed though, most people would guess he would be quicker, but he's not, it's probably because my legs are so much bigger and powerful.
Certified, true story.