Relative sizes in the pictures comes mainly at the editing point. If you crop the image with about the same amount of space around the figure, and then make one of the sides a standard size, it just sort of ends up being pretty close to the same. The use of a klicky in every animal image is to show the relative size of the animal. I felt that otherwise just cropping around the animal an elephant would be the same size as a cat.
I get a little bit of natural light from the sides. There are skylights about 15 feet away on either side. It's not a ton but pictures do tend to be just a bit lighter when done during the day. There are no windows or anything behind or closer than that.
Thanks for the nice comments