Why was PDF file way way too large?

https://forums.adobe.com/people/The+Old+Fart  wrote I was filling in a passport application that required a photo in a pdf file that was no more than 1mb.  The photo I chose was only 507 kb.  I opened the photo  in PSE 11 & just saved it.  The resulting file was useless as it was now 2465KB. I have had to use another program to create a pdf with a usable file size?  Why should this happen?

You did nothing wrong, but in Elements, when you save for pdf format, your only choice (with no options like in Photoshop) is to save in the 'Photoshop pdf' format, not the plain acrobat pdf format.

The Photoshop pdf has a huge advantage because it contains both a pdf and a psd version. It keeps all the layers, texts, vectors and shapes and it can be opened indiferently from PSE, PS or any pdf reader like Acrobat reader. The huge penalty is the resulting file size.

Imagine your file is a pdf/tiff with multiple layers, text or shapes layers and imported smart layers, you'll easily get more than 1 GB.

You have to realize that Adobe does consider that the pdf format (a printing and document format, not a photo format) must be used by the document software which is Acrobat or by the pro editing softwares like Indesign, Illustrator, Photoshop, Lightroom... The result is that the pdf format is hardly supported in PSE. The recent organizer versions only recognizes the Photoshop pdf format. You can no longer manage your old pdf libraries. The option to output pdf slideshows has disappeared.

But, fortunately, you don't need to use free or low cost competitor products to manage your pdfs in most cases (splitting or combining pages...) The widespread solution is to use free 'virtual printers' like the 'print to pdf' Windows 10 option. Instead of 'saving' you 'print to a file'. You get a plain pdf with normal size. This also lets you create multipage pdfs to compensate for the lack of this slideshow option.

What options did you choose when saving to PDF? (If you didn't choose options, try doing so).