You have given absolutely nothing to work with here so this answer is overly generalized.
File size in pdf is coming from OCR, Encryption, compression, resolution.
OCR results in much larger file sizes,
PDF's created for print typically starts around theoretical 200 DPI, but the PDF format shows PPI, not DPI. As pdf actually uses PPI instead.
Stripped down as simply as possible, PDF does not have resolution, only the images inside of them do.
As well, most MFP's have an option to scan as photo or text, text will be much lower file size because it accounts for the content of the document, where photo accounts for the entire page.Tied into the above
A reduction should be seen when enabling background suppression as well, and obviously higher compression, but that depends upon the model.