The more points and paths you'll use, the heavier your file will be. But you'll see this drastic weight increase if you do designs such as maps or "grunge" eroded effects.
The way to fix this is by using the "simplify" command in the "object" menu and then "path". It will help remove some joints, points and corners that might not be so necessary. You might need to also join some paths or combine shapes together with the pathfinder to create bigger blocks with less paths.
One trick I used to do when I had to simplify impossible designs with eroded effects was to export my file in PDF, open it in Photoshop and create a very high resolution 1bit bitmap with it (you can use a normal grayscale too but it has anti-aliasing). Then I would bring back that graphic in Illustrator and then used the "trace" tool to redraw that shape! It was the best way to simplify some portions that the "simplify" tool couldn't really simplify more. Maybe there is a way for you to do this on individual items of your design, especially if it's in color. My trick works better for black and white, or 1-color designs.
If you're using a lot of images or rasterized elements, and if the Illustrator PDF export options to lower your file size isn't enough... You can always use Adobe Acrobat Pro, and resave that file.
This is by far the best way to optimize your PDF since it removes all the invisible data you might not need, will flatten your layers and optimize your images to the resolution you want.
To do this, open our PDF in Acrobat Pro, and go on the menu "file" and then select "save as other..." and "optimized PDF", You will see a few sections on the left side; the "image" one will let you equalize the resolution of all your images. depending on what you need, you can use 300dpi for the color images, 600 dpi for the grayscale and 1240 dpi for the monochrome.
If you don't have anything dynamic or hyperlinks, and don't mind flattening your layers, you can check all the boxes in the next sections. You can also study each of them to see what you want to keep or remove, and make some tests. if you have transparency, try to select the high resolution setting.
And if this isn't enough, you can use in the "image" section the "maximum JPG" compression if you have images in your PDF.
One thing I really like about using Adobe Acrobat Pro is that it will also "crop" your masks and your PDF will be at the right size.
This should really help lower the size of your PDF. If you have a ton of paths though, it won't do miracles but will probably lower your PDF file size.
PS: Unfortunately, your file is not on your DropBox anymore, so I
can't really see what's so heavy with it!