Since you mentioned that you matched resolution (dpi) and color depth what is left to check is PDF Compression.
The software that came with my Canon flatbed scanners gives me the choice between "Standard" and "High" PDF Compression. Other scan software might give you more fine-grained control over PDF Compression (e.g. Adobe Acrobat). Apart from the level of compression there are also different compression algorithms. But usually only more advanced scan software offers the possibility to choose the compression algorithm (e.g. Adobe Acrobat).
So look for the section in your scan programs where you choose the file type (jpg, pdf, tiff, etc.) for your scans. Somewhere there should be the option to choose compression.