File size is what upload forms and email care about. Lowering the JPEG quality from 95 to 80 halves the bytes and is invisible on a screen. This is the first thing the tool does.
Pixel size is what makes an image load fast on a web page or fit a display. A 4000 px photo shown in a 1000 px box is wasted bytes. Choose Fit within 1920 px or 1080 px under Resize to shrink the pixels; the tool resamples in steps and sharpens afterwards so the result doesn't look soft.
When you set only a file limit, pixels are kept unless the limit is impossible at reasonable quality. When you set only pixels, the quality stays high. You can set both.