What are the document's resolution settings? That can be big factor for file sizes, especially if any of the elements have raster-based effects (like drop shadows, glows, etc.). Illustrator defaults to 300ppi for normal sized print documents, but 300ppi would be overkill for any large format project. 72ppi is acceptable for a lot of projects. 25ppi (at full size) is standard for billboard designs.
If all the artwork is vector-based and doesn't have any raster-based effects applied to it the file size and memory burden shouldn't get too big. Still, it's possible to work at a certain reduced scale and then re-size the artwork to full size in the large format RIP application.