My (layman's) understanding of nuclear reactions is that the m in the equation is the difference in mass before and after the reaction, so if two 2H make one 4H

My (layman's) understanding of nuclear reactions is that the m in the equation is the difference in mass before and after the reaction, so if two 2H make one 4He, somehow there is a small change in the total mass that gets converted to energy. Depending on how small this mass actually is, you may still make a fair amount of helium. Do I have the idea right, and its just that there is so much energy in the mass difference that it really ends up making very little? Or am I way off base somehow?