You must have asked Lightroom to create the smart previews at one time (perhaps accidentally). If you don't intend to use them, you can get rid of them with the Lightroom command Library->Previews->Discard Smart Previews ...
However, if freeing up 10GB of space is that important to you, then I think you have bigger problems, and you would be wise to consider getting more hard disk space and even putting your original photos onto this new disk.
...Previews.lrdata folders (not files), are required by Lightroom and should not be deleted. I mean, they can be deleted if they are causing problems (or they're associated with an obsolete catalog..), and they can or will be recreated, but the supported way to minimize storage is:
* delete 1:1 previews, and/or
* delete smart previews
(using Lr menu functions)
Also, if you reduce size and/or quality of standard previews (using Lr preferences), then it won't get so big..
That said, some of us just delete those folders as a quick n' dirty way to "start over" preview-wise..
On a Mac, they show up as files but I see now that they're really packages.
Thanks for the help.
Thanks for your input about how the smart previous came about in the first place. That's helpful and appreciated.
However, please don't take this the wrong way but, as a general rule, speculating about why a question is asked and providing advice that's really not germane isn't constructive.
I do thank you for the relevant part of your response, though.
Smart previews are for times when your photo files are not available to LR. A typical scenario would be that LR is on a laptop and the photos are stored on an external drive, but you are away from home and therefore can't plug in the external. In a case like this the Develop module will use the Smart Previews in lieu of the full sized originals if you want to do some editing (the Library module uses the regular previews). If, however, the photos are on the same drive as LR, a second internal drive or on an external that is always connected, the Smart Previews are not used and can safely be deleted.