'Save As' and smaller file size

[am_dew]

IrfanView 32 bit v4.57 (Windows 10)

When I do a 'Save As' on a file that has not been edited in any way, the saved file size is smaller than the original. The 'Save As' dialog is shown below. Note that the quality is set to 100. Both the original and saved file look identical to the eye. What causes this reduction in file size?

Thanks.

[JendaLinda]

There is always some loss of quality when saving a picture in JPEG format, even if you use quality 100. JPEG is lossy format.

If you need maximum possible quality in JPEG, tick also the option for disabling chroma color subsampling.

[Bhikkhu Pesala]

Note that the quality is set to 100.

In fact, the quality is set to Try to save with original JPG quality. The slider setting is ignored.

Any JPG file with an estimated quality less than 100% will be optimised for size vs quality if the estimated quality setting is enabled. If you save a lower quality JPG at 100% quality, it will get bigger, but will not improve in quality.

The screenshot shows file sizes of the dialog:

1. Saved at original 50 quality (19,866 bytes)
2. Saved again at 100 quality
3. Saved at 50 quality (19,962 bytes)
4. Save as PNG (smallest file size, lossless quality)