Well,with a 100 watt transformer and let's say 15V output,you would have 6.66 amps max.
As for all systems having a short,no they don't.Unless your talking about the filiment in the bulb as a short.The filitment though,is creating resistance,so it's not a short.It acts like a resistor.A short has very little resistance,approx. .1ohm.So,even with a 100 watt transformer,if their was a short(.1 ohms),yes you would still blow the bulbs because then you would be creating 1000 amps of current. (100 watts/.1ohm)
You would create even more current than the 500 watt transformer,because current is inversly proportional to power.(P=IxE)
As the power increases,the current and voltage will decrease,but as the power decreases,the current and voltage will increase.
This is why 110volts AC is more deadly than 220V AC.It carries more current.Current always finds the shortest path to ground.When someone is electricuted,they are the shortest path to ground.They are the conductor.