Why shouldn't you use HF radio while refueling?
The safest way to refuel an aircraft is with everything turned off and the airframe grounded (or bonded) to the fueling system.  This is often not practical (particularly for airliners), so a number of other rules exist to make refueling as safe as possible -- not using HF radio while refueling is one of them.



So why is the HF radio singled out? Power, combined with the antenna design factors you mentioned.  

With small radios you can easily light a fluorescent lightbulb (incidentally this is why radio geeks are such great fun at parties). HF radios can put out several hundred watts of power - this makes them more likely to generate a spark that can jump the gap between say a refueling nozzle and the refueling port/tank, igniting fuel vapors and ruining everyone's day.
HF antenna designs which use the aircraft itself to radiate part of the energy can further increase the chance of a spark.