Using a US/UK voltage transformer at low end of appliance power range
Firstly 500/600 watt is a generality. Transformers are not that accurate. When you see a 500 watt rating that means it will stand at least 500 watts. Also transformers as you said come rated in steps i.e. 500, 750, 1000. You can always use a larger size as long as you have room to fit it. The actual rating usually comes from the wire size that the transformer is wound from. As for the 500/600 rating that usually derives from either the 50 or 60 cycle power that it is run on. It can in some instances come from the base supply voltage i.e. 110 vac or 120 vac and or a combination thereof. So you see it is not that critical or exacting, just make sure a transformer is always as big or a little bigger than the heaviest peak load that you intend to pull from it.