Many brides want a big cake though they don't have many people to serve. I already know about doing dummy cakes. What are some other ideas? If you have some pictures that show a "small" cake looing larger, share it!
cakes with pillars are taller than stacked cakes.
Using cake plateaus, pedestal display stands, acrylic cake stands (the stand alones) add height to a cake.
It's more of an old fashion look, but using that Tuk-n-Ruffle under your cakes can give the illusion of a larger cake. When using the tuk-n-ruffle, you can put a 10" cake on a 12" plate ... with the ruffle taking up the 2" space difference, the cake doesn't look lost on the larger plate. You can get the same effect by using greenery (ferns, larger leaves) tucked under the cake.
As indydebi mentioned, using greenery or flowers is great around a cake. I also sometimes (if the site allows) place tealights in high-sided glass votives around the cake, about 10" away from it, safely back from the table edge. It gives a nice soft lighting, and also makes the cake area look larger without being busy looking.
Another thought, for the cake itself, is to use RI or gumpaste lacework or flowers that stand "off" the cake. Several examples are in this book -- http://www.wilton.com/store/site/product.cfm?id=3E30A675-475A-BAC0-525BB0AADDF3EB94&fid=E6A7ADE9-423B-522D-F1F716093FC52F69