On my most productive (in my own estimation) brownfield days in recent memory, the codebase would generally shrink by several hundred LOC.

I also find that a huge factor in my code production rate on brownfield projects might not have much to do with me, because it's factors like, "Is the code well-documented, easy to understand, and backed by tests that make the intended behavior clear? Or do I have to start by burning days or weeks on wrangling with Chesterton's Fence?"

And, on the other side of it, when is documenting and cleaning my own code to guard some future maintainer from that situation vital, and when am I burning a day of my own time to save someone else only an hour in expectation? All I know for sure in that situation is that, if my manager is assiduously counting LOC, ticket close rate, anything like that, then game theory demands that I should never bother to spend an extra hour on making it more maintainable if I expect that the cost of that decision will be born by one of my teammates. The 10X rockstar developer at a previous team of mine taught me that lesson in a rather brutal manner.