EDIT: I have now read your replies to other peoples responses. I see you have already acknowledged the point. Consider this response retracted as redundant.

Flip. Push. (All else being unrealistically equal.)

Pushing is going to get you prosecuted for murder in most jurisdictions,

You are fighting the hypothetical. Note that my response refrained from fighting the hypothetical but did so explicitly and acknowledged the completely absurd nature of the assumption that there are no other consequences to consider. That disclaimer should be sufficient here.

Or suppose you construct the hypothetical so there is no legal comeback: still, why do you want to push a fat man off a bridge?

Because I want to save 5 people.

It takes energy, you could pull a muscle, he could notice and hit back or pull you over too etc. etc.

Again, I chose not to fight the hypothetical. As such I refrained from opting out of answering the moral question by mentioning distracting details that are excluded as considerations by any rigorous introduction to the thought experiment.