I have been working on a project lately, and as I've drawn more on the layers the memory usage has been going up slowly. I haven't added any layers during this time, just drawn more on existing ones. So, why does painting more on a layer increase it's size in the memory it needs?
I have been getting the memory usage going over 7GB momentarily and giving me a ⚠ on the memory monitor.

The details on the info box are kinda confusing. First it says that image size is 5.2GB, but then it says there is 2.5GB worth of image data. If I crop 150px of excess image on each side, the memory usage drops to 2.4 GB on image size and 2GB on image data. Why does image size drop that much from such a small crop (considering the full size of image)?
This might help for a while, but I still need a few more layers later on so the size surely gets back up to where its now.

Anyway, is there anything I can do to decrease the memory usage? Unfortunately resizing the image to smaller size is not possible, because the image has a lot of detail and losing it can make making sense of some parts impossible.

A layer take up as much and as little memory as it needs to hold all of its content. And empty layer takes up around one pixel (technical detail), but on the other hand if you draw a lot outside of the canvas, it is still there and can take up memory. Maybe just try to crop your image to the canvas size? It wilL cut off all the data outside of the canvas if you have any, which is what I suspect happens here.

Anyway, is there anything I can do to decrease the memory usage?

I will let others address the question of how to reduce memory usage. To increase the memory available for your work in Krita, you can do this:

Settings > Configure Krita > Performance

Increase the Memory Limit to 100%. That size is equal to, but not the same as, the amount of RAM.

Krita will need to be restarted for the Memory Limit change to take effect.

An explanation is here.

tymond wrote:Maybe just try to crop your image to the canvas size? It wilL cut off all the data outside of the canvas if you have any, which is what I suspect happens here.

That did seem to work, in cutting the memory usage to 2.5GB. I'm not sure how some data ended up outside the canvas since I don't remember ever moving anything fully or partially outside the canvas.