cs3 cropping tool

[bkamps14]

So ive been using the cropping tool in cs3, the one where you can chose the size of the crop (eg. 6in x 4in). My original file is dramatically being reduced in file size (eg. 6mb to 1mb) even when im only slightly cropping). Im not really understanding why? Its not actually cropping the photo to that size as i can enlarge it to 100% and its bigger than 6x4.
Any helpers??

Thanks in advance

[DanielLane]

what file type are you starting with and what are you saving as?

[bkamps14]

Opening and saving as jpeg