Resizing vs. Compressing: What's Actually Different
Resizing changes an image's pixel dimensions (width and height), directly reducing file size by having fewer total pixels to store. Compressing keeps the same dimensions but reduces file size by encoding the image data more efficiently, at some cost to visual quality if pushed too far — the two are independent adjustments that are often combined for the largest file size reduction.
These two terms get used interchangeably in casual conversation, but they act on different properties of an image and it's worth knowing which one actually solves a specific problem.
What resizing changes
Resizing directly changes how many pixels make up the image — a 4000×3000 pixel photo resized to 1200×900 has genuinely fewer pixels to store, which reduces file size roughly in proportion to the reduction in total pixel count, independent of any compression setting applied.
What compressing changes
Compressing keeps the same pixel dimensions but changes how efficiently that pixel data is encoded — a more aggressive compression setting produces a smaller file at the same dimensions, at the cost of some visual quality once pushed past a certain point, as covered in the article on why compressed images look blurry.
Why combining both is usually the most effective approach
For the largest practical file size reduction with the least visible quality loss, resizing to match the actual needed display dimensions first, then applying a moderate compression setting on top, typically outperforms pushing either adjustment to an extreme on its own.