Someone recently asked me what was happening with the preview file when the user is deleting or dereferencing an image. A good question that needed some monitoring. What I have found is the following...
When importing an image, if the user has requested the creation of the 1:1 preview, LR creates two preview files which are named after the global_id of the image in the database (the global_id is an identifier that never changes as long as the image lives in the catalog - it cannot be the file name because the image can be renamed or because two different images in different folder can have the same name). The name of the preview is the result of the concatenation of the global_id with another value representing the preview, the "digest". So you get something like that : CA4C526B-898B-4D58-8DBC-B035356BB141-5e7425c24caa33e18b8e99baf98dfe6b.lrprev (global_id-digest.lrprev).
The previews are stored in a subfolder which name is based on the 4 first digits of the global_id, here <catalog_folder>\nomCat\nomCat Previews.lrdata\C\CA4C. As mentioned above, two files are created, one for the standard preview and one for the 1:1 preview, the latter being much bigger.
Now, if I delete or dereference an image in LR, only the standard preview is deleted. The 1:1 preview will only be deleted if a catalog optimization is triggered or when LR does its 1:1 previews cleanup (each day, each week or each month, according to the catalog preferences).
So if you have set this setting to "Never", you may find yourself after a while with a lot of useless 1:1 previews.
Question : Why doesn't LR delete the 1:1 preview immediately when an image is deleted or dereferenced?
I think you have a good point. But it seems that the 1:1 previews are tied to that option in the catalog settings. Personally, I never use 1:1 previews. They only have impact in the library module, and I do all my work in the develop module. If you think this is an issue that should be addressed, it would be good for you to express your concern on this website:
I admit, this is a real issue only for those users who have set the "purge frequency" setting in the catalog preferences to "Never". However, why not do things correctly in the first place? If the image is no longer in the catalog, there's no good reason for keeping the preview on disk.
I will report this on the feedback site.