What Is Lossy Compression? The Tradeoff Behind Smaller Files
Lossy compression shrinks a file's size by intentionally discarding some of its data — a genuine, permanent loss, but one that's specifically designed to be as visually or audibly unnoticeable as possible, which is why it's such a widely used tradeoff for photos, video, and audio.
Why deliberately throwing away data is a reasonable idea
The key insight behind lossy compression is that human perception has limits and biases — the eye doesn't notice extremely fine detail as readily in busy, complex areas of an image as it does in smooth, simple ones; the ear doesn't perceive certain sounds clearly when they're masked by louder, nearby sounds. Lossy compression targets exactly this kind of perceptually less-important data to discard first, preserving the detail that's actually noticeable while cutting the detail that mostly isn't.
Why it achieves such dramatic size reductions
Because lossy compression can genuinely remove data (rather than just finding a more efficient way to store the same data, as lossless compression does), it can achieve far more dramatic size reductions — often producing a file a fraction of the size of a lossless equivalent, which is why formats like JPG and MP3 became so central to making photos and music practical to store and share at scale.
The tradeoff is a dial, not all-or-nothing
Lossy compression typically comes with an adjustable quality setting — pushed lightly, the discarded data is genuinely hard to perceive; pushed aggressively, visible or audible artifacts start to appear (blocky patches in images, a "underwater" quality in over-compressed audio). Finding the right point on that dial for a specific use case is really the whole practical skill involved in using lossy compression well.
Why the loss is usually a fine trade
For most everyday uses — a photo viewed on a screen, music played through typical speakers or headphones, a video watched on a phone — the discarded detail from moderate lossy compression is genuinely difficult for most people to perceive, while the file size savings are substantial and very noticeable in terms of load time, storage space, and ease of sharing. The exceptions are professional contexts (archival photography, audio mastering, video intended for further heavy editing) where preserving every bit of original detail is worth the larger file size.
Compress Image and Compress Video both apply lossy compression directly in your browser, letting you compare the size-versus-quality tradeoff at different settings before committing to one.