I have heard this before and in most cases it is a lie created by manipulative or despising partners to blame, guilt or cause worry within their spouses. Sleep talking can happen to anyone and I have witnessed a case in which a flatmate laughed in his sleep but this case seems is far fetched and too focused on his personal interests to be true. The probability of laughing in your sleep and offending him is low. He is most likely venting frustration on you by making you worried and guilty. On the off chance he is telling the truth, a simple question can be answered. The night before you sleep talked, did you think or focus intently on his weight and does thinking about make you laugh? How do you feel about his weight?