If you’re using RW8, then I’m not sure this “notion” that size somehow matters when it comes to resources.
I have the options set correctly (the default) dragging something like PDFs images or even videos into the RapidWeaver resources makes a copy of the item.
Yes, it makes the project “file” larger, but a RapidWeaver project file is a special kind of folder called a package. You can right-click on the project file in finder and select to view the contents of the package, and it looks like any other folder.
Inside RapidWeaver resources, you can also right-click on any resource, and there’s an option to show in finder. You can see the resource is nothing more than another file on your Mac.
Now I’d be the first to admit in older versions of RapidWeaver like RW6 and RW7 Resources had many issues and developed a bad reputation. I used to “stay away” from using the built-in resource folder like many users, but RapidWeaver 8 reworked the entire way resources work.
There are some reasons why you might not want to use resources, like file management and publishing, but I wouldn’t worry much about file size.