The external render folder in ZBrush can get large in file size because it stores all the rendered images and associated data files that are created when you render your ZBrush project.
When you render an image in ZBrush, it saves not only the final rendered image, but also various intermediate files that are used to generate the final image. These files include things like render passes, depth maps, and texture maps, among others.
Over time, as you render more and more images, these intermediate files can accumulate in the external render folder, causing it to grow in size. Additionally, if you're rendering high-resolution images or using a lot of complex materials and textures, this can also contribute to larger file sizes.
To help manage the size of the external render folder, it's a good idea to periodically go through and delete any old or unnecessary rendered images and associated files that you no longer need. You can also adjust the settings for your renders to produce smaller files, such as by reducing the resolution or using lower-quality settings.
On Windows, the folder is at C:\Users\Public\Documents\ZBrushData2023\ExternalRenderer
On Mac OS, it is at /Users/Shared/ZBrushData2023/EternalRenderer