Why is image reduced in file size after processing?

[Billy Budd]

When I process or adjust images in XnView and elect to overwrite the original image, the size of the file is greatly reduced--from 3 or 4 MB to 500 or less KB. Why? Is there any way I can avoid this result? Thanks

[xnview]

Do you have checked option/Write/Jpeg/Quality?

[Billy Budd]

Thanks, Pierre. Yes, that works fine when 'save as' is selected but when overwriting the original image after processing, the new file size is decreased dramatically.

[xnview]

Thanks, Pierre. Yes, that works fine when 'save as' is selected but when overwriting the original image after processing, the new file size is decreased dramatically.

You don't have the same file size by 'save' and 'save as'???

[helmut]

Just to make sure: When JPG compression is lossy, that means when saving the image the image is not saved 1:1 but modified a bit for better compression. In Wikipedia the compression is described in more detailed.

Topic 'Lossy? Lossless? 256 colors? This is confusing!' also contains valuable information.