No, it's not always true. Spark plug sizes depend on the amount of room in each cylinder. Most V8 engines have cylinders that are of similar size to a 4-cylinder engine's, and have similarly sized spark plugs. Some 4-valve per cylinder engines have smaller spark plugs as there's less room in there for the plug, but this isn't always the case.
If you want small spark plugs, look at a motorcycle engine - many have plugs which look like toys. They're expensive though!