Warning: this may be offensive to Christians. For the record, I am not against Christianity at all.
When Christianity came about, they started using symbols from other religions to try to convert and consume peoples religios beliefs. The crusifix is an adaptation of a Pagan symbol (I don't know it's original meaning). The Pagan symbol was a cross but with all arms the same length, as opposed to the bottom arm being longer. By using this symbol, the Christian church could say "See, we're the same as you. You should follow the rest of our beliefs too." Obviously it wasn't quite that simple, but that was one part of it.
Interestingly, Alex and I were wondering through St. Pauls Cathedral in Melbourne where Alex noticed a number of the older crosses did have arms of equal length. She asked the people at the kiosk about this and all they said that it was the same thing.
