There are many factors that affect the size of the PDF. Your problem may be in the way the PDF creation libraries handle font embedding, specifically:
- "Embedded subset" means that part of the font's metrics, like glyph widths, are included in the file.
- If the font is not embedded, presumably it is loaded by the reader from the system, reducing the size of the file.
If the PDF is already small (only has one page, little text and no images), embedding fonts may make a relatively big difference on the size of the document. Still, in absolute terms, an embedded font shouldn't take a lot of space.
Another factor you should check is compression. PDF is mostly a plain-text stream, but it usually comes in compressed form. Try opening both PDFs in a plain text editor and see if it's readable or gibberish. The gibberish (compressed) form will naturally take less space.
Finally, you can inspect the objects the PDF file is composed from using the many PDF inspectors out there, for example this one (I just googled it up, no guarantees it'll work as expected).