Does Increasing File Size Reduce Image Quality? (Explained)

No — increasing a file’s size by padding it cannot reduce image quality, because the padding is written into metadata areas that image decoders skip. The pixels that come out of the padded file are bit-for-bit identical to the original. Quality is only ever lost when a file is re-encoded, which is a different operation.

Two different things that both "increase file size"

Padding adds inert bytes to a container. In a JPG those bytes go into COM (comment) marker segments; in a PNG they go into ancillary tEXt chunks; in a PDF they go into an unreferenced stream object. Every one of these is defined by its format’s specification as something a reader may ignore, and readers do ignore them. The image data is not read, rewritten or recompressed.

Re-encoding decodes the image to raw pixels and compresses it again with different settings. That is what happens when you move a quality slider, and it is a lossy round trip for JPG: each pass discards a little more detail, whether the resulting file is larger or smaller than the one you started with.

Why a larger re-encode can still look worse

This surprises people. If you take a JPG that was saved at quality 60 and re-save it at quality 95, the file gets bigger — but it does not get better. The detail thrown away at quality 60 is gone, and the second pass faithfully preserves the damaged version while spending more bytes doing it. You end up with a larger file that looks fractionally worse than the original, because it has been through two lossy compressions instead of one.

This is why "increase quality" tools that simply re-save at a higher setting are misleading. You cannot recover discarded detail by asking for more bytes.

What upscaling actually does

Resampling to larger pixel dimensions is a genuine change: a 600×400 image redrawn at 1800×1200 really does contain more pixels, and for print or large display that can be what you need. But the new pixels are interpolated from the existing ones, so the result is bigger rather than sharper. It will not reveal a licence plate or a line of small print that was not legible before.

If your goal is to satisfy an upload form, padding is the better choice: exact, instant, and provably lossless. If your goal is a physically larger image, use the resolution mode and accept that it is enlargement, not enhancement. The difference between file size and resolution is worth reading if this distinction is new.