How to reduce the size of an Illustrator file

The easiest ways to reduce the file size of Illustrator documents that contain linked graphics is to uncheck the option to "Create PDF compatible file" in the "Save As" dialog box.

To make the file PDF compatible, Illustrator embeds the fonts and raster graphics into the illustrator file. This will allow the file to be previewed with Acrobat, the Finder, or by another application. This feature may be might be necessary if you're going to place the illustrator file into InDesign, but if you work and output directly from illustrator then the PDF compatibility is unnecessary.

If the file is purely a vector drawing then I would recommend allowing it to save it as a PDF compatible file because  it would only embed the fonts and the increase in the file size is nominal. The only way to decrease the file size of vector drawing further is to delete unused swatches and symbols but the reduction in size is slight. I deleted the unused swatches and symbols in a test file and the size was reduced in size from 741KB to 676KB. Both of those file sizes are tiny compared to the sizes of today hard drives.

EXPERIMENT
As an experiment, I placed five jpeg files into an illustrator document. Each jpg was approximately 5mb in size. The sum of all of the file sizes was 22.6MB. When I saved the illustrator file with the PDF compatible option checked the final file size was 88.6MB. Since the files are linked they would have to be stored with the illustrator job, which means that this job would take up 111.2MB of space!

I then resaved the document with the PDF compatible file option UNCHECKED and the size of the Illustrator file was reduced to 1.2MB which results in 23.8MB of space used for this job. That's a savings of 87.4MB! This is especially important if you work on hundreds or thousands of jobs because it will consume a large amount of disk space over time.

IF YOU NEED TO SHARE AN ILLUSTRATOR DOCUMENT AS A PDF
My recommendation is that if you require the file to be viewable in a PDF application such as Adobe Acrobat, then just save a copy as a PDF with a compressed setting such as "Smallest File Size". The same file saved as a PDF with the smallest file size option, created a file that was 123KB. You may have two files instead of one but the overall storage will be greatly reduced. The compression settings can be adjusted if the graphics in the PDF are too pixelated or blurry.

CAVEAT
One caveat is saving without the PDF compatibility means that the file will no longer have a preview, so you won't be able to preview it in the Finder. Also, if you try to place the illustrator file into another document, the preview will be replaced by a text that states:

"This is an Adobe Illustrator File that was saved without PDF Content. To Place or open this file in other applications, it should be re-saved from Adobe Illustrator with the "Create PDF Compatible File" option turned on. This option is in the Illustrator Native Format Options dialog box, which appears when saving an Adobe Illustrator file using the Save As command."