An Aperture library needs more storage than the storage used for the original image files you imported. Aperture creates additional thumbnails for faster browsing and additional previews for sharing the images with the media browser. But together these additional files should not add up to twice the size of your original import, the size of the library should perhaps increase by 25%. How much additional storage is required will depend on the size of the original files.
Have you edited your photos in an external editor? External edits can blow up the library size considerably, if you save the edits back as TIFF files or PSD files with layers.