Not a large K program, but a company I used to work for maintains a large, open source q framework https://github.com/AquaQAnalytics/TorQ which is used in many large investment banks and hedgefunds.

Funnily enough, the framework is actually a more expansive version of a tick system developed by Kx (the company that makes kdb) https://github.com/KxSystems/kdb-tick

The Kx one is incredibly concise. When I first started working with it, it took me a while to figure out what was going on.

The larger K/q programs get, the more they tend to look like "normal" code, but you still see a lot of these clever one liners hidden away in there.