Why do some voltage reference ICs (LM385B) have 8 pin packages where 6 pins are NC?
There are multiple issues at play here, actually.  Die size, as mentioned, is one of them -- a SO-8 or even a TSSOP-8 can accommodate a larger die than a SOT-23 can, although modern references are available in SOT-23 packages these days (the Intersil FGA parts come to mind, specifically).

Other reasons include mechanical stability for high-accuracy references and the need to provide post-packaging trim access due to packaging process induced shifts in the reference voltage.  LTC AN82 discusses these problems in more detail.