Here are some real real world examples:
Taken with 20mm lens:
Taken with 540mm lens (400+1.4x converter):
Make sense? If I'd used that 20mm lens on the sheep he'd be a tiny spec in the frame- the size of one of those rocks in the landscape shot. And vice versa, had I used the 540mm lens on the landscape one of those rocks would be about all you'd see in the pic.