Vector graphics is a compact way of storing simple drawings. And I always thought that the most compact .pdf file you can get from Illustrator, is achieved with the "Smallest file size" preset. However, I have recently discovered that a .pdf saved from Illustrator with this preset or any other I have tried so far is very large compared to what Inkscape can produce.
For example, here is the same simple picture drawn in Illustrator and Inkscape and saved in the .pdf format with minimal settings. It is easy to see that while the Illustrator .pdf is 37.3 Kb, the Inkscape .pdf is 3.2 Kb, which is 10 times smaller! An enormous difference!
So, the question is why the "Smallest file size" setting of Illustrator makes such huge files and can Illustrator .pdf files be made much smaller (because I have found myself recently making figures in Illustrator, saving as .svg and making a .pdf in Inkscape)? And what on Earth does Illustrator write in those .pdf's?
Here is an .svg picture which can be open in both Illustrator and Inkscape in case someone wants to try. And I put her a .png picture just in case my uploaded files are no longer accessible: