Yep, it didn't do the feat justice since Oda chose a more asian-looking mountain structure for this island. However, in one panel you can see that these mountains are actually large and tall. Like that:
This panel showcases the mountains at the background in comparison to the actual hills which are clearly significantly smaller than these. And Burgess lifted and threw one of these. And in this panel:
See the bottom left corner of the panel, Law destroyed this mountain and you can perfectly see the other mountains as well as a part of the island's shape. There is a clear distinction between the hills/forests and the mountains.
I wouldn't even mind the fact that Burgess' body is visible in front of the carried mountain. That's just Oda's way to show the character's position. We got plenty of these examples in Dressrosa and Wano.