The first thing that springs to mind is that Adobe might have tweaked the size of the auto extent growth (or some other setting that affects the size of the file allocations) in the underlying SQLite DB that creates the files in the folder you are looking at. This might have caused overall size of the DB to increase under some circumstances, especially at the beginning under sudden load.
Just to be clear on the process:
If you had LR4 upgrade your LR3 catalogs, all that happens is that LR4 "steals" the previews from the LR3 catalog and renames the previews folder to match the LR4 catalog name. Thus if you were to examine the contents of the Lightroom catalog folder immediately after the upgrade you would find the original LR3 catalog but with no associated previews folder, and the LR4 catalog with an associated previews folder.....and because its a simple hi-jack of the LR3 previews folder it is of course the same size as the old LR3 folder.
If you then open the LR3 catalog in LR3, because no previews folder now exists a new one is created and (unless you then ask LR3 to rebuild previews for all the images again) LR3 will create standard previews as you bring pictures into the grid/filmstrip. In this scenario, obviously the newly created LR3 previews folder will be far smaller than the former previews folder that is now labelled LR4.
I have seen many posts since LR4 was released complaining about the size of the LR4 previews folder in comparison to LR3, and I just think in many cases there was a lack of understanding about the process.
Thanks for the help clvrmnky and jim1403.
Jim, your explanation is dead on. There is some interaction between the V4 lrdata and the old lrdata. Just for fun, I mounted a couple of my backup drives dated prior to version 4 and there is very little difference in the catalog file or the lrdata files between versions.
I have now removed LR3.4 and removed the associated files (I do have a back up) and now full speed ahead with LR 4.1 RC
Thanks again for the help,