Those are basically marketing terms used by the various companies to describe the size of their servos. GWS, for example, uses the terms "Pico" and "Naro" (Asianized Nano) to describe their two smallest servos. If you know your science, you'd know that pico and nano are prefixes in the metric system, like milli and centi (e.g. millimeter and centimeter). A picometer is smaller than a nanometer, hence a Pico servo is smaller than a Naro.

The only way to know what terms mean is to go to the manufacturer's website and look them up. There is no standard naming convention for servo sizes. For example, can you tell that a Hitec HS-50 is comparable to a GWS Pico by looking at their model names/numbers? Nope. You need to look up their statistics on their respective manufacturers' websites.