Lightroom has three 'data' items:
- Lightroom Catalog
- Lightroom Preview database
The Lightroom Catalog holds information about the images, plus pointers to where they are on disk, and usually small previews. There are no images or RAW files in this Catalog.
The Lightroom Preview database contains larger previews for each of the images cataloged in the Catalog. These are the previews that take a bit of time for Lightroom to create during import, and are optional. The images are not in the Preview database, though 'previews' of the images are kept here.
The images themselves are not part of Lightroom at all, and can be stored nearly anywhere. Lightroom uses these as a 'master' and does not change them. It refers to them to generate on-screen images, and when doing an export, it reads the image in.
So, given this, 'removing' a RAW file from the catalog simply takes that file metadata and small previews out of the database. Since it is just info on the image, and not the image itsefl, each image has relatively minimal impact on the size of the Catalog, but they do add up. Since the image is not part of the Catalog, removing it won't reduce the Catalog size by the size of the image, only by the size of the metadata (date taken, ISO, f stop, etc). You can remove photos from the Catalog, but this doesn't do that much, unless you remove a significant portion of your images.
If you are looking to reduce the amount of space Lightroom consumes, look to the Preview database rather than the Catalog. You will find the Preview database is much, much larger than the Catalog. You can safely delete the Preview catalog, though there will be performance hits to Lightroom as a result.
If you want to reduce the size of the Catalog, the best way is to split the Catalog into many smaller catalogs, typically by year: so create a '2015' Catalog, with just images from 2015. You can do this by selecting all your 2015 images, and then 'Export as Catalog'. Backup your old catalog then delete it from the machine, leaving just the '2015' Catalog, and simply double-click it to launch it in Lightroom.
Obviously, deleting the image files themselves will make the largest impact, but I assume you are not seeking to delete the images themselves.
Based on your comment, your specific issue is identifying those RAW files that are on your hard drive within the image folders, that have been removed from the Lightroom Catalog. Here is how to identify those photos:
- At the folder level, within Lightroom (left hand side within the Library Module), choose a Folder, then right click.
- Select 'Synchronize Folder'. If there are any photos on the hard drive that are not in Lightroom, the dialog box "import new photos" will have a number behind it: e.g (4). This means there are 4 images on the hard drive that are not in Lightroom.
- Be sure to check the box immediately below this one: 'Show import dialog before importing'.
- Click the button to Synchronize. This will bring up the import dialog, showing all the images your Removed, but didn't delete. You can make note of them and delete them manually, or, better yet, just import them. If you do this for the top level folder (say the entire hard drive or the "Photos" folder, or each year, you can find them all.
- If you Import them, Lightroom creates a Collection called 'Previous Import' immediately after an import. After you import the images, simply select the 'Previous Import' collection, select all the images, and click Delete, removing them from Lightroom and the hard drive.
For future reference, the best practice is to simply tag images as 'Rejected' (press X on the keyboard for each photo). This tags the images as 'to be deleted'. When you are sure you want to delete photos, you can choose Photos>Delete Rejected Photos, to remove them from the catalog and delete them. This also allows you to remove them from the view, as you can filter on this 'rejected' tag.