It's all in the title. I'm staring at a tree as I'm walking down the sidewalk. The tree appears to be about 4 inches tall. I walk twenty steps closer, and it looks 5 inches tall. I walk another twenty steps, and now it looks 7 inches. Its appearance is, perceptively and to me, growing on a steepening curve.

Why is this?
It's something I noticed today so I felt like posting here to get an answer. I'm guessing it has to do with our depth perception.