That 1GB of RAM per 1TB of disk is a recommendation from the ZFS documentation, but let's remember the audience, and the feature sets enabled when we talk about it. In particular, that suggestion stemmed from running in a configuration with file deduplication enabled and heavy amounts of caching, which ZFS is made to take advantage of.

The high memory usage profile definitely isn't from an extra 4 bytes on a pointer, but from design and features of the filesystem.