Modeling the chain-length distributions (CLDs, the molecular weight distributions of individual branches) in a polymer system can be exploited to obtain information on the underlying (bio)synthesis mechanisms. Such a model is developed for starch (a highly branched glucose polymer), taking into account multiple isoforms of the three types of enzymatic mechanisms contributing directly to the CLD: propagation, branching, and debranching. The resulting CLD is given by two parameters and can thus be represented by a point in a two-dimensional phase diagram. The model implies that all native-starch amylopectin CLDs are confined to a line in this phase diagram, an inference supported by fitting data for a wide range of plants. This gives new ways to classify mutants and suggests useful directions for plant engineering (e.g., which isoforms could be targeted to give long branches, which are nutritionally desirable).