Burgess is not stronger (over all) than King or on his level. He just possess more physical strength. Physical strength is just one type of power or strength
Burgess has showed more physical strength or lifting strength than any other character in the manga. That is including the Yonkos.
Whether you want to call it a giant hill or small mountain Burgess did pull it out from the ground, lifted it above his head, and threw it. It is an impressive feat. We have never seen Big Mom or Kaido do the same with their physical strength but that doesn't mean Burgess is on their level either.
It's not just about having physical strength is about how well you able to use it in battle.
Mihawk shouldn't be as physically strong as Kaido or Big Mom. Yet, he easily cut through a giant frozen wave, bigger than the mountain Burgess lifted, with a casual attack from a distance. That attack is stronger than Big Mom's elbaf attack. That is because how well and skillfully Mihawk able to apply his physical strength to his sword attack or techniques.
Zoro 3 billion world attack lifted a mountain size Pica golem into the air. The golem was larger than the mountain.
Burgess is close to Dressrosa Zoro but still weaker than him.