What could cause one Mallard duck to become freakishly large???

My back yard ends at the shore of the Patterson/Passaic River, which is home to many different types of ducks, geese, and swans. The most common of these fowl is the Mallard duck. I regularly feed these birds stale bread and have recently noticed that one of the ducks has been growing significantly larger then... show more My back yard ends at the shore of the Patterson/Passaic River, which is home to many different types of ducks, geese, and swans. The most common of these fowl is the Mallard duck.

I regularly feed these birds stale bread and have recently noticed that one of the ducks has been growing significantly larger then the other ducks in the river.

While its coloring is correct for a Mallard, and it seems to be correct in its proportions, it’s about three times larger then any other adult male Mallard.

What could possibly be causing this single Mallard to grow to such an unusual size???
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