I think the logic makes sense. Presumably the thinking is that zombie processes take up 64KBytes apiece because there aren't enough of them, in general, to make it worth doing the work to make them take up less.
The reason the memory is retained is so that it can be reported to the parent process why the process died (e.g. normal exit (error code 0), or failure, or killed by signal, etc), and retain enough information for simple diagnostic purposes. It is for that reason that page table mappings (for the little memory that is retained) and private data (for process exit status etc) is kept.
But zombie processes aren't everywhere. Typically they exist briefly and then get cleaned up. So even if they could take up 4 bytes, nobody's bothered writing the code to make them do that, because what's the point? If they take up 64KBytes it's just not a very pressing problem.
The point is that there are always tradeoffs, whose resolution is affected by how important an issue is perceived to be.