I'm sure that most people that were not fit to work were killed immediately when enter most camps but were overweight prisoners generally better off against starvation?
This isn't so much a historical question as a biological one, that is, the question of how various body compositions react to malnutrition.
Furthermore, given that the portion of the population of 1940s Europe who could be deemed obese was likely incredibly marginal, we don't really have enough points of historical data (nor a controlled or uniform environment) to assess your question at all, I'm afraid, besides picking out possibly one or two anecdotes.
This is a good question. There should be some stats, given that there were 70k+ survivors of Nazi concentration camps.
I would be interested in additional breakdowns: by vocation, educational level, physical attractiveness, work assignment, home country/province, etc.