Are you scanning them to a Mac as suggested by the support document you posted from? Or If so, what app on the Mac are you using? Pages? Something else?
Or are you scanning them directly on the iPhone? If so, what app on the iPhone are you using? Notes? Something else?
By definition the iPhone scans are in the highest quality possible. Scans through other means would be of lesser quality or possibly compressed. The iPhone produces larger images and as such larger files. With said, I just scanned a document into Pages on the Mac and then exported it as PDF and it produced a 3.4MB file. Easily sendable over email.
How are you trying to transmit it exactly the requires such small files?
For example Pages on the Mac can reduce the file size of the document before you export it as a PDF by going to the File Menu ➜ Reduce File Size.
And when you Export it as a PDF, you can change the image quality, from Best to Better or just Good to also reduce the file size.
Outside of that, you can always use online services to compress the PDF file such as Adobe's own free PDF compressor: https://www.adobe.com/acrobat/online/compress-pdf.html