Many times it's supply and demand. For example, many airlines fly the 757 only to smaller markets in Europe... such as Continental flying them to cities such as Oslo or Birmingham, England. The larger planes go to bigger cities such as Paris or London-Heathrow. More people want to go to the bigger cities so they need bigger planes.
by the # of passengers they expect to get! 757 is really about as small a plane as you want to fly across the pond...there are some 737's & A319s, but they are all Business Class.
If they can fill a bigger plane, they will fly a bigger plane.
What's available, what fits the expected number of passengers, what they can make the most money with.