[cogman10]

There were also a lot of practical limitations on the hardware that need to be recognized. These weren't devices that could store megabytes of data for code or memory. Not only that, but the compilers were also a lot dumber (by necessity). So, optimizations you'd normally leave up to the compiler (like inlining) you instead did by hand.

[jandrese]

This part is frequently lost on people. Bell labs developed a multi-user operating system that supported multiple people logged in at the same time on a machine with 64 kilowords (144kB) of storage.

Later development was done on a machine that supported a max of 4MB of memory and had to allow for hundreds of simultaneous logins. Keeping the code compact was a high priority, even over usability in some cases.