From what I have just heard from a tugboat captain, it is not the length that is the problem, but the width of the newer ships, compounded with the 50 foot extensions of the Manhattan piers that were added to aid in the loading of the ships in a more efficient manner. It's a tight fit to get two large ships between the two piers and have tugs in between.
On a side note, yesterday when I spoke to him on this subject, he commented that the newer ships really do not need the tugs anymore, but they have at least one to standby in case of a problem. He jokingly said that one tug, like the one he operates, would not be that effective on a ship with the sail area of say the QM2 or the new Epic. He also said that Cunard was very particular, insisting on coverings over the tug bumpers to protect the paint from scuffs.