I used to have a regular-width MLD bivy. I'd definitely recommend the wide option, and I'm a pretty skinny guy. With a 76-77" girth, I can sit up comfortably inside for eating, foot care, map juggling, etc. That makes for a much more livable arrangement.
The main disadvantages are the greater airspace to keep warm, and greater convective heat loss if you toss and turn alot - but neither are especially noticeable compared to a narrower cut, IME.