[zubat]

I believe the main size optimization here comes from having only teletype and batch processing interfaces. When you do that, the surface area of the UI, and hence the amount of supporting code, drops tremendously. These early tools also skimped on error messages and checks, so the user experience was generally one of confusion and catastrophic error, despite being small and simple. You really needed the documentation to have any hope of understanding an old system.

Now we have operating systems that go out of their way to automate away everything and present it in real-time with multitasking and custom graphical elements everywhere, while also supporting many more protocols and hardware interfaces - wireless networking, GPU APIs, etc. The biggest growing pains seem to be past - things went from simple and stable to complex and unstable in the 1990's and then to complex and stable but insecure now. There's a lot of room to mature all of these features, but there aren't as many novel ones.