Why should it?  After all, you're sort of abusing ctypes by repeatedly creating Struture types over and over again.  C structures that you might want to wrap with these types are fixed and known at the time of programming, so there is never a need to create the same Structure type twice.  Thus the set of Structure subclasses created during the live-time of a program is both, fixed and small, so there is no need to limit the cache size.