This is hard to explain, but "big lug" is a general term of affection for any man, no matter what his physique. It is something a woman would say to him as an affectionate term while at the same time trying not to embarrass him or make him feel less "manly." For example, if your husband or boyfriend does something particularly thoughtful for you, something which a male might be embarrassed for his friends to know (say, he brought you breakfast in bed), a woman might express her appreciation by saying, "Thanks, you big lug." It is a way of expressing your thanks and affection without being overly romantic.
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