Why is the amperage lower for higher wattage devices?
You likely answere your own question -- wattage is Voltage times Amperage.  What you might not know is that in higher wattage situations, they'll typically run a higher voltage (eg. 220V) circuit, so it's possible that the 30Amp circuit is actually getting more total power than the 45Amp circuit.

But those aren't your only considerations when sizing the breakers -- motors tend to draw about 50% over whatever their rating is when they first kick on.  You can either place them on a high-rated circuit, or you can put them on a 'slow trip' fuse (or circuit breaker) that won't trip the instant it sees a surge; naturally, this is not something you want to install in a wet area where you'll need a GFI breaker.  Because of this, particularly if they're going to be installed in wet area (where you don't want to use a slow-trip), you'll see higher than expected breakers for anything with a compressor, pump, or other motor.