I admit that I am not familiar with ARIN, but if it is anything like APNIC (Asia Pacific) then those big Class A Allocations are protected as legacy allocations. As such they are not required to give the legacy allocations up and its questionable if the RIR could even revoke them.

ARIN can go over the pool of post 1997 addresses they have allocated, but I think you would find much smaller unallocated blocks.