How to clear iPhoto cache

Hello fellow iMac users,

I need help on how to clear the iPhoto cache. This is the scenario when I import images to iPhoto through Flash Card from my Canon Camera. After importing images from Flash Card, I transfered the images in iPhoto to external hard drive and once it has been transfered I deleted the images in iPhoto. I noticed that iPhoto Libary size is gaining MB, from 190 mb to 735 mb. So I viewed iPhoto Library to its Show Package Content and I could not find a cache folder for this matter, similar with iPod Photo Cache Folder. But I observed that on the Masters folder, Previews Folder and Thumbnails Folder have those images that has been imported. Since I am afraid, for now, to move inside the three folders (masters, previews, and thumbnails) to the Trash so I need help to which any of those folders are best to delete?