As stated, the wattage rating just has to do with the durability of the components used. The W1000 and W5000 are actually the same exact antenna for instance. The W5000 just uses thicker wire in the coil than the W1000.
Antennas are TUNED for transmission. A "perfect" antenna for 11m CB band is 102 inches. A straight piece of metal, 102 inches tall. This is your best performer for a mobile application. You need a solid mount for it, I do not advise using a magnet mount.
Now, antennas that are SHORTER THAN 102" still CAN work on 11m CB band...they work by adding INDUCTANCE. At the bottom of a base loaded antenna, like the W5000/W1000/W500/Lil' Wilson, there's a thick plastic cap. Underneath the cap is a coil of wire.
The coil of wire adds INDUCTANCE to the antenna thus lengthening it electrically, but allowing you to use a shorter stinger (the metal whip portion).
As you can guess, there are disadvantages. For one, loaded antennas are more complex than a 1/4 wave resonant 102" whip. That coil is one more thing to break, or fall out of tune. Two, you lose wattage to heat. Antennas shorter than 1/4 wave (102") will produce dB of LOSS. You Lil Wilson is only probably about 30% efficient. So 100w into it, you get maybe 30 watts that are actually radiated. They are also LESS SENSITIVE to received signals. A bigger piece of metal picks up weak signals easier.
So there's your radio theory 101 lesson for today.
In short, if you want a cheap magnet mount antenna that works : Get a Wilson W1000.
If you want maximum performance, buy a 102" whip and mount it in a Hustler C32 ball mount with a 6" spring heavy-duty.