Some people just have physically larger muscles. They may not necessarily be any stronger or more powerful. They will be heavier though, so will require more effort to move. Someone like Chris Hoy has massive quads because of his body type , and lots of work to maximise the muscle strength and power.
But most blokes with big ‘quads’, yes, it’s just body fat. Some people naturally carry more body fat, and combined with big muscles, can have very large quads, but again, they won’t necessarily be any more powerful than skinnier chicken legs. I have the latter, and when I was a bit younger, had no problem out-climbing and out-sprinting other lads with much bigger legs. It’s all about the efficiency, not the size of the engine. 😉