Why is my WebP or AVIF bigger than the original?
1 min read · Last updated June 2026
It can, when a source image is already heavily compressed — there is little left for WebP or AVIF to improve, so the modern copy can come out the same size or slightly larger. When that happens, SlashImage skips the variant rather than serving a bigger file, so it never reaches your visitors. You keep your already-optimized image; you just don't get a modern-format copy that would not have helped.
Why it happens
Modern formats like WebP and AVIF find compression gains by working on image detail — particularly in photographs with lots of colour and tonal variation. An image that is already heavily compressed has had most of that compressible detail removed, so there is little left to exploit. In that case a WebP or AVIF copy can come out the same size or slightly larger, and SlashImage drops it rather than serving the bigger file.
What you see in the SlashImage column
When this happens, the SlashImage column shows Already optimal. That means the plugin looked, found no meaningful saving, and left the image as it was. It is not an error — it means your image was already well-optimised before it arrived.