What about Um_nik ?

Because many of the top competitive programmers in the world are from Russia and Russians are generally thinner than the others.

Perhaps it is because of their diet and their eating habits. It has nothing to do with competitive programming...

PS: If you want to become like them you can read this

I think it is a combination of a few things. First, competitive programming is a very stressful activity and top programmers are in this state consistently for extended periods of time. That in itself consumes a lot of calories, even if it's not physical exercise. For comparison, you can read about how many calories chess players burn in one day at a tournament.

Another factor is that competitive programmers can be sedentary for long periods of time, and not getting enough exercise to maintain healthy muscle mass.

Of course not all top competitive programmers are the same and I'm not an expert on this topic, but these can be a few contributing factors to the trend.