Pixel dimensions describe the raster grid. File bytes depend on format, image detail, transparency, encoder and quality as well.
Dimensions control layout
A 1200 × 800 source resized to 600 × 400 has one quarter as many output pixels.
Encoding controls bytes
Two files with identical dimensions can differ greatly in bytes. ImageResize reports the actual generated Blob instead of predicting it.
Verify the generated file
After resizing, compare the reported output dimensions and format with the requirements of the receiving website or document. Open the downloaded file there, inspect important edges and transparent areas, and keep the original until the result has been accepted. Check the preview at its natural size as well as inside the destination layout. If text, fine lines or a logo matter, zoom in and compare them with the source. A smaller byte count is useful evidence, but it does not by itself prove that the visual result is suitable.