According to this from Ian Lyons: http://forums.adobe.com/thread/358039
"1:1 previews will not be deleted unless your chosen standard-sized preview is less than half the size of the full-sized preview"
I had a >100GB previews cache, and doing Discard 1:1 Previews did not reduce the size of .lrdata. I just changed my standard preview size from 2048 to the smallest setting and did Discard 1:1 Previews on a bunch of photos: LR is taking much longer to do the operation than the first time. It is only about 1/3 done and the size of my .lrdata (as shown in Finder) is already down to 63GB.
Perhaps this will work for you.
Ian does say that in some cases the space won't be freed until a while after LR is closed and re-opened, but in my case the size shown in Finder started going down immediately.
I'm not exactly sure what Ian means by "half the size". Probably means that if the width of the full size image is not at least double that of your standard preview size, then LR won't actually delete said 1:1 preview (perhaps it keeps it instead
of the standard size preview???).