The Fat Man and the Trolley

This entry in Wikipedia describes a very interesting series of moral dilemmas, the first of which is this:

"A trolley is running out of control down a track. In its path are 5 people who have been tied to the track. Fortunately, you can flip a switch, which will lead the trolley down a different track to safety. Unfortunately, there is a single person tied to that track. Should you flip the switch?"

Most people would answer 'yes', given that saving five people is "better" than saving one. An interesting tweak on this:

"As before, a trolley is hurtling down a track towards five people. You are on a bridge under which it will pass, and you can stop it by dropping a heavy weight in front of it. As it happens, there is a very fat man next to you - your only way to stop the trolley is to push him over the bridge and onto the track, killing him to save five. Should you proceed?"

Now the survey participant might get a little squirmish. Why is that? The moral calculas is the same (saving 5 versus saving one).