In developing countries there can still be a lot of poverty for people. Sometimes poverty can lead to weight gain (usually in wealthier nations), and sometimes to low weight (usually development regions).
Developing countries are still developing, often there isn't yet a high standard of living and many people may still be very poor. Here in the wealthier countries poor people often have access to poor food, cheap junk food etc... which can make them fat. In a developing country they may not have this.
"An estimated 146 million children in developing countries are underweight - the result of acute or chronic hunger (Source: The State of the World's Children, UNICEF, 2009). All too often, child hunger is inherited: up to 17 million children are born underweight annually, the result of inadequate nutrition before and during pregnancy." http://www.wfp.org/hunger/who-are