They probably tone it down because it isn't as important as the first one. Also, it seems that Shinra didn't really care about it being destroyed. After all, the trio was cornered on that bridge outside of the reactor, showing that Shinra was waiting for them. Even the president himself made an appearance. And they obviously didn't disarm it. Maybe they wanted it to be destroyed. So the people of Midgar would hate Avalanche and love Shinra more. But it still seems strange that they would let a reactor simply be destroyed........hmmmmmmmm.