Why is the Lightroom preview folder so big, even after purging the 1:1 previews?

Question

1:1 or full-size previews take up a large amount of HD space, often many 10's of GB's. This is why building them on demand is an alternative to building them up front, and why you have options for auto or manually purging them. That said, it may not always be possible to delete 1:1 previews - more later. 
Standard-sized previews are based on the pixel size option you chose in Catalog Settings. You have 4 choices: 1024, 1440, 1680 and 2048. If all you want is standard-sized then choose a size equal to or greater than your screen size. Lr also builds smaller versions at the same time. These are used for the thumbnail, 1:2, 1:3 and 1:4 zoom levels. Adobe call this the "preview pyramid". 
1:1 previews will not be deleted unless your chosen standard-sized preview is less than half the size of the full-sized preview. For example, you have images with pixel dimensions of 3072 by 2048 pixels (6MP camera such as Canon 10D) and have selected 1680 pixels for standard-sized previews. Since a 1680 pixel preview is more than half the size of the original the full-sized preview will be kept, even after using the purge 1:1 preview command. Had you chosen 1440 pixel for standard-sized previews then the full-sized would have been purged (i.e. 3072/2=1536, which is greater than 1440). Now take the example of a larger camera, say a 8MP camera such as the Canon 1DMKII with an image size of 3504 by 2336 pixels. With the standard-sized set at 1680 pixels the full-size will again be purged (i.e. 3504/2=1752 pixels, which is greater than 1680 pixels). 
Another thing worth mentioning - Lightroom will usually delay purging previews for deleted images until after the application has been closed and reopened, even then it might take a few minutes to clear the purged previews. The idea here is to ensure the previews don't get deleted whilst there's still a chance that they may be needed (e.g. you removed images by mistake then use Undo command to reinstate them). 
Hopefully above explains why Lightroom doesn't behave as you expect. If it makes your head spin, think how much mine spun when writing it.

Answer 1

The chance of an answer is inversely proportional to the length of the posting. ;)
You do not indicate what your preview setting was while you accumulated a 12Gb preview folder. Nor do you indicate how many images are in your catalog.
Given your screen size of 1280 (laptp?) I would set the preview size to 1440 since this is still far less than half of your D3 images.
Cropping - I'll safely guess - has no effect on your preview size choices.
I confess that preview sizing and purging is confusing. I avoid the confusion by shouting "more disk, mama".