On Windows, what data (format) goes to the printer almost never is the same as the document format you create and edit or receive from someone.
(An exception may be an XPS document viewed in an XPS viewer being sent to an XPS-capable printer...)
The document format may be Excel/XLS(X), Word/DOC(X), PowerPoint/PPT(X), PDF, HTML, EPUB, whatever. No printer understands these formats. A printer requires some specific format(s) to be able to consume it and print it on paper.
The data transfered to the print device (and before transfer being spooled in a local folder) is different: On Windows the locally spooled file format is EMF ('Enhanced Meta File') or (O)XPS ('(Open) XML Paper Specification'). But these are are also not suitable for a printer. Also, it is not the original file which gets sent to the printer and then is no longer on your PC: it's a copy of the file, and that copy needs to be converted to suit the printer. It is the job of the Printer Driver to accomplish this conversion:
- a PostScript printer wants PostScript
- a PCL printer wants PCL
- an ESC/P printer wants ESCP
- an RCPS printer wants RPCS
- a raster printer wants raster data (many possible variants)
So some printer drivers will convert all pages to hi-res raster data, which takes a lot of bytes. And this is what you observe as the "increase" of file size, but mis-interpreted it: because your original document is still un-changed (in format as well as in file size).