This is normal for dSLRs.

The image you download from the media card is your original image file. I never change this. I archive all my original image files just like I used to archive my negatives.

The if I want, for example, an 8x10 I set my crop tool to 8x10 @ 300dpi, make the crop and "Save As" with a different name. I prefer to crop vice resize.