Man, this is digging back to my high school knowledge, but I think this is a plausible answer:
The water level will fall. When the nuts and bolts were in the boat, they displace their MASS in water, since the boat effectively makes the metal bouyant. When they fall in, they will displace their VOLUME, which since metal is denser than water, will be a smaller amount of water, making the water level fall.
So hows about it? Am I a genius?
The smaller bridge is stronger. Doubling the dimensions would exponentionally increase weight while not increasing "strength" per diameter of structural member, etc. of the bridge.