As a single true value in an array the size of the number of building types (>6000), for each unit type (>1000). i.e. a single true value in 6 million false values. How many innocent and meaningful bytes of information did this monster carelessly force out of the cache? We may never know, but one thing we can be sure of is that its days are numbered.
If only one unit has the ForceBuildings tag used in the xml then ForceBuildings only needs 6000 values because those arrays are only initiated if they are needed.