Pros/cons of big databases vs smaller databases and RDS

Hi-- to keep it short, I feel like I've generally heard the larger your DB
is, the less efficient it might run, likely due to disk I/O. Maybe I'm
terribly mistaken in this perception.

We have two DBs, one primarily accessed by humans and systems, which is
~1TB in size, and aggregates most of what we store in raw, longer format on
a second DB that is about ~6TB in size.

As we consider plans to migrate to RDS, we've talked a lot about
combining the two as more and more the case is querying the larger DB and
wanting data only available in the smaller DB.

Of course, we can solve this by copying things back and forth, but we're
also thinking: why not just one big DB?

Anyone have any experiences with a similar project, and especially any
technical configurations that might be beneficial in using RDS?

Appreciate it.

--
Wells Oliver
wells(dot)oliver(at)gmail(dot)com <wellsoliver(at)gmail(dot)com>