I might ask why you created Smart Previews?
They are only of use when the original image files are not available.
An example would be- your Original images are on an external hard-drive that is NOT plugged in. Then LR would use the Smart Previews to allow development of the images.
Or you might want to select some images to work with while you travel with a laptop- you would select images, Export as a Catalog, include Smart Previews, then this is the laptop catalog- Import as catalog when you are back to desktop.
Smart Previews are down-sized images in DNG format with a long edge pixel dimension of 2560, so yes, they will take up some memory! but they are big enough to export JPGs suitable for prints up to about 8x10in.
If you decide you do not want the SPs you can selectively remove them from the Menu > Library > Previews > Remove Smart Previews. Or you can 'Globally' remove ALL SPs by deleting the Smart Previews Folder that will be next to your Catalog location.
Perhaps you have misunderstood what Smart Previews are. They are 2560 pixels wide DNGs and are used as editing surrogates for the original Raw files if they are unavailable; for instance, when you are away from the external. So to be used in editing they have to be fairly good sized and of high bit depth, not just compressed jpgs like the regular previews. But when the originals are on-line, the SPs have no function. LR won't use them, so they just sit there, taking up space. For this reason it makes little sense to automatically make them for all imports, but rather to build them as needed for files that you will want to work on away from base and later, when the editing is done, to discard them.
Thank you both. I do understand what SPs are for and really liked the idea of having the option of editing any of my photos on my laptop without having my external drive connected. I know I had seen in a handful of places that the SP file is supposed to be small. Here's a link to such a place I saw this, https://helpx.adobe.com/lightroom/help/lightroom-smart-previews.html In the section named "Advantages of Smart Previews." This is not a big deal, it would be a nice option if it didn't take up too much space.
Are your ORIGINAL files JPG or RAW files and how big in pixel dimensions are these files if they are JPG. The big savings in smart preview size listed in the document you referenced is probably only for RAW files. Your 5.5gb and 3.5gb sizes don't seem correct if they are RAW files.
They are useful tools, but my point is that they have a limited and specific function. Other than for image editing ("developing") while you are away from home-base, they serve no purpose and it is highly unlikely that anybody would always be working away from home/office or that he would always be re-editing all of a catalog of tens of thousands of photos. Thus, it seems pointless to build and archive more SPs than are needed for a specific period - trip, day, etc. - and since you are not saving huge numbers of them, the space issue seems a nonstarter.
Also, like Bob, I wonder what your originals are. I just made an SP from a 25 MB CR2 file from a 5DII and it was 1 MB. A second one from an 11 MB CR2 from a 40D was also 1 MB. Because they are lossily compressed DNGs, their size will vary according to content and ISO, but in my experience they run from 1 to 2 MB.
Working away from home is not the only thing Smart Previews are useful for! I can think of several ways they are useful for working at home, one of which is extremely useful.
One example is sending photos for others to work on. I sometimes do this when outsourcing my editing to Lavalu, for example. Rather than FTP thousands of huge RAW or DNG files, I can just FTP a much smaller catalog and associated Smart Previews file.
The other thing, and what makes Smart Previews so awesome in my mind, is they can tremendously speed up editing!
I agree 3.5 GB for a smart previews file seems large for 5.5 GB worth of photos though. I suggest deleting the Smart Previews lrdata file and building Smart Previews over again.
Yes Smart Previews are useful but they are specific to Lightroom and are not image files that can be displayed or used in other applications.