A low average number of children was not an explicit goal during development of CWE-1000. However, the CWE team sometimes paid special attention to large categories to see if there were additional patterns that might provide a greater degree of granularity, which had the effect of reducing the average number of children. In addition, creation of a more abstract entry might cause entries in other parts of the hierarchy to be shifted to that abstract entry. However, some areas of CWE-1000 clearly need additional granularity, such as CWE-693 (Protection Mechanism Failure) and CWE-668 (Exposure of Resource to Wrong Sphere), which both have 26 children.
It is interesting to note that while one of 7PK's original goals was to use "seven-plus-one" kingdoms for psychological acceptability to humans, this only applies to the top-level kingdoms; the input validation kingdom contains 28 items. This may be due to the limited depth of the hierarchy. Vulncat, which expands 7PK, adds a top level based on individual programming languages; each level is then broken down by the 7PK categories. The average number of children still appears to be higher than those in CWE.
The hierarchy in 699 seems deeper than it really is, because of the small number of roots at the top. It does not really begin to branch out until a couple levels down into the hierarchy. So, it is fairly equivalent to 1000 in terms of depth.
Both CWE-699 and CWE-1000 average more than 1 parent per node. In the case of CWE-699, this is expected due to the large number of categories in use. For the Research view, while mutual exclusivity is a goal, further work is needed.