Of the several engineers I've managed at my startup, the ones who wrote the most code got the most stuff done. It's a correlation and there are exceptions, but it's generally the case, at least for new software. A project won't complete itself without a lot of code being written or modified. Although I think total number of PRs is possibly more useful as a metric than total LOC (assuming neither are being gamed).
This is sometimes true for early stage projects. There are exceptions, sometimes you can have an engineer creating a library or a framework without any need. An existing library would have done a better job faster, but that engineer just wanted to make a new one. As a result you can have a massive framework, known only by that engineer in which that engineer is very effective. But no one else is. And overall it is a liability, if compared to an existing framework.
In general, the more code you have the more code you have to maintain and massive codebase churned out by a single engineer is more often a liability, not an asset.
I know people who write quickly huge amounts of very poorly thought of code that will be buggy and will have a bunch of completely useless functions, reimplemented functions that already existed within the project and similar stuff.
You might be unaware that you are not such a great manager.
