File size: KB or MB
File size measures how much storage the encoded image uses. It affects upload limits, email attachments and download time.
Dimensions: pixels
Dimensions describe the image width and height, such as 1200 × 800 pixels. They affect how much detail and display size the image can support.
Why resizing and compression are different
Resizing changes the number of pixels. Compression changes how efficiently those pixels are stored. A large photo may need both steps to meet a strict upload limit.
A practical workflow
If a form requires exact dimensions, resize first. If it also requires a maximum KB value, compress the resized image. Use the Image Resizer and then the Image Compressor.
Do not optimize blindly
Always check the destination's actual requirements. A 100 KB limit does not tell you what dimensions or image format are acceptable.