The guy on the right isn't any larger than the guy on the left. They're just standing in an Ames Room, a space carefully constructed to play tricks on the mind. The room is deeper and taller on the left, and your perspective is limited by the peephole the room's creators built. But, if you didn't know all that, and you were looking for a big, strapping, theoretically-evolutionarily-well-adapted man, you'd probably pick the guy on the right.
And that's exactly the kind of trickery male bower birds base their dating lives upon.
This is a bower from the one of the avenue species of bower birds -- those who build a long avenue out of sticks, with a court at the end made of stones, shells, bones and bits of colored plastic. The female stands outside the avenue (where the photographer was lying to take this picture) and looks through it to the male bower bird who is dancing around on the stones at the back.
The funny thing about this picture is that to us the stones look like they are all a similar size, but they are actually arranged with the largest ones in the back, and the smallest ones in the front. If you switch the positions of the stones, the males will move them back into the opposite size gradient within three days. One of the possible reasons the male bower bird creates this Ames court might be to make himself look bigger in the front of the court when compared to other objects placed in the back next to the bigger stones.
Creature Cast: Perspective of a Bird