Blade shape, construction method and materials all come into play, but i think the most important consideration is the intended use of the blade. A machete is primarily for chopping smaller brush with a loose grip in a flaccid arm. The power and efficiency comes from the design of the blade and the speed with which it moves through the air (force=mass x acceleration).
When using a knife as a chopper, the energy comes primarily from the user's muscles, especially the core muscles. Even the best heavy chopping blade is not designed to reduce its caloric impact on the user (since they are heavy in the hand without the added benefit of length).
Also, the definition may be more ethnographic than technical. We as outsiders put values on items and create categories that the native user may not. For example, in the mountains of southern mexico and northern Guatemala (where I have spent time) the Coffee farmers use the term "cuchillo largo" or large/slow knife depending on your translation to describe what most of us would consider a full blown machete. These users are placing their tool within the framework of knives, rather than differentiating them based on their intended method of utilization.