Not all tables need even the capacity for 2^30 rows, much less 2^31, or 2^63. If you have a reference table with things like timezone information, color schemes, etc. and are using anything other than a SMALLINT (2^15), you're probably wasting space.

As to the maximal 8.6 GB mentioned, that's not nothing, _especially_ for RAM. Disk is cheap, but RAM isn't. If you have a smaller instance – say, an r6i.xlarge on AWS (4 vCPU, 32 GB RAM), that's 1/4 of the entire DB's memory.