Americans' expanding waistlines were a factor in the capsizing of a Baltimore water taxi that killed five persons in 2004, a trend that has led to other deadly transportation accidents, the National Transportation Safety Board said yesterday. The board said the water taxi tipped over because excessive passenger weight made the boat too unstable to withstand a sudden gust of wind. The NTSB said the Coast Guard underestimated the "tippiness" of the 36-foot Lady D pontoon boat because it used the results of a stability test designed for a different type of vessel.