Can two species thrive on a single energetic resource? While the competitive exclusion principle predicts that one in the pair should go extinct, it may occur that an organism releases partly metabolised molecules in the environment, securing an ecological niche for a second organism in a specialisation process called metabolic cross-feeding. Here we investigate how evolution may favor the waste of a useful resource using a model that considers how a cell packed with proteins may be less efficient, hence favoring a shortening of metabolic pathways in order to reduce cell packing. Our model indicates that such specialisation only occurs under restricted conditions. Incidentally, this makes the signatures of cross-feeding, such as which metabolites are preferentially involved, quite predictable.