Have you verified that you have not changed the type of file your saving as the default. AS a basic JPG file takes up less space than a find JPG file and a TIF or RAW type file is even larger. Also the resolution increase the file size across the board.

At the RAW setting if your camera has one, a 8G card has room for about 666 images (file size 12 mega each).

Example from my 6 mega pixel camera Basic JPG about 1 mega, Fine JPG 2-3 mega, RAW 6 mega.