I actually don't think it's a ton. Go count the lines of code of one of your small projects. I have something I spun up in the evenings over a few days and it's 2000 lines. It feels like nothing.

Tech giants have thousands of engineers working on these projects for many years and delete and rewrite features on a regular cadence. The fact that Messenger is only ~500 times bigger than my project is kinda weird to me. Mine has 1 user, it has 3 billion.