The frame usually works fine for a ground. However, consider what troubleshooting will look like when your widget isn't working right. Where is that widget grounded? Can you find it? can you reach it to troubleshoot and fix it? A common ground at the firewall, with a dedicated wire from everything back to ground adds a pretty negligible amount of weight to a small airplane, and ensures you aren't relying on any dodgy fasteners that might be contaminated with paint, oil, polybrush, oxidation, etc.
Also, current is a conserved quantity. All the current that leaves the battery comes back into it. All the current that goes into your starter comes back out. So your ground wires should be the same size as your power wires. They are handling the same amount of current.