> Seems like they just got so much bigger that we don't even recognize them as a department store. Walmart, Costco, Home Depot, et al. are just variations on the theme.

That's not an issue of size; Walmart, Costco, and Home Depot are smaller than department stores are.

And department stores have gotten smaller over time; they used to be much bigger than they are now, which you can see by visiting a flagship store.