The immutable source ratio is 3:2. Multiplying 600 by 800/1200 gives exactly 400.

Pixel count falls to one quarter

Both axes are halved, so area changes from 960,000 to 240,000 pixels.

Bytes are measured later

Format and content still decide the generated Blob size; dimensions alone do not.

Reproduce the result

Choose any ordinary image with the stated source dimensions, enter the example setting with aspect lock enabled, and generate the file. The displayed output dimensions must match this example; byte size can differ because pixels, format, quality and browser encoder also matter. Repeat the example with PNG and JPEG to see that identical dimensions do not imply identical bytes. Download each result and reopen it before relying on the browser preview. The source remains the reference if a receiving service alters the file again.