Hi, I have been making some custom themes for my browser but for some reason the crx files are really large (16-20 mb) I was wondering if there was some way I could change this so it is more download friendly, thanks for your time

Thats freakin huge!
Open up the resulting crx with WinRar or something and make sure there isnt a file in there called "Cached Theme.pak" if there is then make sure you delete that file before packing or zipping your theme.
What sort of images are you using?...do you understand the basics of themeing?...such as that most image tile (repeat) so you dont allways have to use huge images, plus most images only need to be a certain height and after that its just excess.
Have a look at your files and delete anything thats not needed and upload it so we can have a look and give you better advise.

Omg, I am actually ashamed of myself right now, I opened it up with winrar and what was causing the size was a folder of images I was using for background lolz, totally noob. I know the basics of theming *sadly the folder thing probably says otherwise lmao* >.< I was so shocked by the size of it I must have lost all my senses O.O Anyway now that the issue is solved I will start uploading themes here ^^ and I can't thank you enough for your help!!