http://www.ncbi.nlm.nih.gov/pubmed/3958810 [1986!]
"Mice from groups 3-6 [restricted intake] exhibited mean and maximal life spans 35-65% greater than for group 1 and 20-40% greater than for group 2. Mice from group 6 [most restricted intake] lived longest of all." Brackets mine.
"The total monthly calorie intake was the same for the FMD and control diet groups, indicating that the effects were not the result of an overall dietary restriction."
There are a lot of possible factors that may be involved. From changing behaviors, lifestyle, lack of disease, larger buffer before weight loss becomes an issue etc.
Edit: It should be noted that this still a fairly low BMI compared to most middle aged Americans.
Considering diet is so important for managing the disease being light weight is probably correlated with a greater focus on health.
That said, it seems likely that reducing weight would have a net positive impact on heath and it's other risk factors which are being improved.