I often wondered why fixture wires (usually 18g or smaller) are allowed to be tied to a 15 or 20a ckt. Would'nt there be a potential for the hot wire heating up if it came in contact with ground before the circut had a chance to blow?

Same principle as allowing your lamps to be fed with #18 or 20 zip cord. The load is definitely limited to the appliance (100 watt bulb, ballast, etc.)and there has been very little problem with any "hazard" associated.
Also, don't forget the ballasts are protected with a thermal element, so think of it like the 10 and 25 foot tap rule.

In the case of a fault, probably gonna burn itself clear before any real damage can occur, but that is why you have to be very cautious with fixtures above flammables.

For a piece of history, flourescents did not use to have Class P (Protected) ballasts when I came around the trade, and lotsa people would not allow them in their barns, houses, etc. because they had been known to cause fires.