Image Compression vs Resizing

Both compression and resizing make an image file smaller, which is exactly why they get confused for each other. They do it by changing two completely different things, and picking the wrong one either wastes effort or damages the image more than necessary.

Compression changes how efficiently the same pixels are stored

An image compressor keeps the same pixel grid, the same width and height, and changes how that pixel data is encoded. Lossy compression (JPG, lossy WebP) throws away some visual detail a viewer is unlikely to notice; lossless compression (PNG) finds redundancy in the pixel data and stores it more efficiently without discarding anything. Either way, the image dimensions don't change. A 3000×2000 photo compressed hard is still 3000×2000, just encoded more efficiently, or with a bit of blur/artifacting at high compression.

Resizing changes the actual pixel grid

Resizing reduces the number of pixels the image actually contains. A 3000×2000 photo resized to 1200×800 has genuinely fewer pixels to store, which is usually a much bigger file-size win than compression alone when the image is being displayed smaller than its original resolution anyway (a thumbnail, a blog post image, an email attachment that doesn't need to be print-resolution).

Why resizing usually wins for file size

Pixel count scales with the square of the dimension. Halving both width and height doesn't cut the pixel count in half, it cuts it to a quarter. That's why resizing a huge photo down to the size it'll actually be displayed at almost always saves more space than compressing it at full resolution, and does it without introducing the visible softness or blocky artifacts that heavy compression can leave behind.

When compression is still the right tool

If you need to keep the image at its original dimensions, a print-resolution photo, a design asset someone else will resize later, an image where you genuinely don't know the final display size, compression is the only option that reduces file size without permanently discarding resolution you might need later.

The practical order

If a large file needs to be small and you know roughly how it'll be displayed: resize first to the dimensions you actually need, then compress the result if it still needs to be smaller. Doing it in the other order, compressing first, then resizing, wastes the compression pass, since resizing recalculates the pixel data anyway.

Tools

Resize Image to change dimensions, then Compress Image if the resized file still needs to be smaller, both run entirely in your browser.