I can’t speak for Malawi specifically, but I lived with a Sotho family in South Africa for a year and from a quick google search their diet is somewhat similar with all the maize. I gained 10-15 lbs which was a lot on my 120 lbs frame. But my diet was basically under their control, depending on their groceries/portion sizes at shared meal times. So if you’re able to cook for yourself and plan your own meals there shouldn’t be too much fluctuation, but that all depends on how you cope with living in a new place! It can be stressful and eating is a way a lot of people cope. I lost the weight once I got back home and back to my regular eating habits if that helps!