Imagine it like cooking at home vs takeout.
Engine-light is like cooking at home, the engine (restaurant/market) does as little as possible. For the most part you have to do everything yourself to create a meal (the game).
All the sauces need to be created from basic little ingredients, meal has to be properly cooked and spiced.
Engine-heavy is takeout. You just choose which food you want. That is great if they have exactly what you want, otherwise there are problems.
At the time this was getting decided, they were in the middle. To continue the analogy: They ordered burger patty, it was done from one side, then they had to manually do the other. The salad was cleaned, but as whole head(?).
That meant, you had to order from the restaurant - which needed all the equipment and took resources to make everything; at the same time, you still had your kitchen all dirty and spend time on making your own meal.