You can't shrink an OS after it's been installed. You can use BBSAK to remove cod files. But just be careful and know what you're doing because you can really mess your phone up.
Probably easier/safer to just reinstall it and shrink it in the process. I did that to mine a couple hours ago. Easy. got more crapola out I missed the first time.
I used a combination of these to increase my file free to 45mb's on a 8330 curve. Since then I have loaded numerous themes, apps, etc. And at fresh boot have a comfortable 28mb.