The term Nose Tackle doesn't refer to a 'big fat DT' as you put it. Though the nose tackle is usually the biggest player on the line, it doesn't always mean he weighs 350 pounds. Kyle Williams is a NT for the Bills and he weighs 295, less than some 3 technique guys. It's about where the player lines up, and what his assignment is.
When a guy lines up as a 0 technique (head up on the center; typical for a 3-4 NT) or 1 technique (guard's inside eye; typical for a 4-3 NT), he's typically refered to as a nose tackle. Scheme makes no difference.
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