and do I really need an aperture database if aperture can use the iphoto library database?
If you do not have any other libraries, that you would want to unite with your iPhoto libary, there is no need to convert the library by importing. The format is compatible.
iphoto previews folder (1.82 Gb)
aperture previews folder (123.3 Gb)
how do I reduce the size of the previews folder?
How many image versions do you have? That is an amazing difference in size!
You change the size of the "Previews" in the Aperture Preferences, the "Previews" tab. Generally it is recommended to set the pixel size of the previews to the screensize of your largest display and to pick a good quality setting. Compare the settings in the Preferences panel with your screensize.
After you changed the settigs for the Preview size and quality, delete the previews and update them.
To delete the previews select your images in the Browser, ctrl click, and use the command "Delete previews". Then check, if now the size size of the Previews folder is smaller.
Then create new previews by using the command "Update Preview". If that is not available, hold down the options-key and select "Generate Preview".
Regards
Léonie