I guess because we're a "bigger is better" type of society. Look at nearly all car models...since downsizing in the late 70s/early 80s, they've become progressively bigger in all classes.
One thing that pisses me off though is clothing sizes, which have become progressively larger. I have slightly older but decent "medium" shirts, which are now the same size as newer "small" shirts.
It's even worse for polyester athletic wear. I keep that stuff almost forever because it never wears out, but I've added to my collection over the years. Older, tighter fitting medium shirts are smaller than the baggy "small" shirts I recently bought. Dammit, I'm not getting any bigger, why should my clothes? :|