Compress vs resize: what's the difference?

The short answer

Resizing changes the pixel dimensions of an image (e.g. 4000 × 3000 → 1600 × 1200). Compression keeps the pixel dimensions the same but stores each pixel using fewer bits, usually by discarding visual information the eye barely notices. Most real-world workflows use both.

What resizing does

When you resize a 4000-pixel-wide photo down to 1600 px, you're throwing away 84% of the pixels. The remaining pixels are recomputed from neighbours using a resampling algorithm (bicubic, Lanczos, etc.). Done well, the result looks almost identical at the smaller display size — and the file is dramatically smaller.

• Best for: images that are stored or displayed larger than they need to be.
• Watch out for: resizing too aggressively makes text and fine detail look soft.

What compression does

Compression encodes the same pixel grid using a more efficient representation. JPG and WEBP use lossy compression — they discard detail your eyes are unlikely to miss. PNG uses lossless compression — it never throws information away, but the savings are smaller.

• Best for: images that already match the target display size.
• Watch out for: aggressive JPG quality (below ~70) creates visible "blocky" artefacts.

When to use each

Use resizing when…

• The image is much larger than where it'll be displayed (most camera photos).
• You're publishing to a platform with a known max width (Instagram 1080 px, blog 1600 px).
• You need to send via email and the file is over the attachment limit.

Use compression when…

• The image is already the right size, but the file is bigger than necessary.
• You're optimising a PNG screenshot or graphic that doesn't have many colours.
• You're converting a PNG photo to a smaller WEBP or JPG.

Use both when…

• You want maximum savings (almost always — first resize, then compress).

A real example

A 12 MP photo straight out of your phone is roughly 4000 × 3000 px and 5–10 MB. To use it as a blog hero image:

1. Resize to 1600 px wide. New size: ~1.5–2 MB.
2. Compress by exporting as WEBP at quality 80. New size: ~150–250 KB.

That's a 95–98% reduction with no visible loss at the size your readers will actually see the image.

How iPhoto Resize handles this

Our resizer always lets you pick both: a target dimension and a format/quality. Set the width, choose JPG or WEBP, drop the quality slider to ~80, and the tool does both jobs in one pass — entirely in your browser.