The file size is too large.
The size of the file will depend upon many factors. The physical size of the document and the amount text / graphics on each page are factors that will affect the resulting file size. These factors cannot be changed.
To produce the smallest scan file for a given document, use the lowest resolution and Black and White mode, and scan the smallest area possible. Using the opposite settings (scanned in the highest resolution, in the Color mode, and scanning the largest area possible) will produce the largest-sized file.