It might be a surprise, but the cache is not storing thumbnails, but the full raw render files in .exr format. What you see in the Recent Renders palette are thumbnails generated from the exr files, but those thumbnails only exist in memory while Poser is running, not anywhere on disk.

The full raw renders are cached in case you want to re-render areas (saving a great deal of time if the areas are small), so the new area render gets composited with the prior cached, full render. Multiple area renders can be composited and saved out as a single file in a format of your choice. Deleting the cache will free up a lot of disk space if a large number of renders are contained, so some like their cache cleaned frequently.

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