  The notion that any physical quantity is defined and measured relative to a
reference frame is traditionally not explicitly reflected in the theoretical
description of physical experiments where, instead, the relevant observables
are typically represented as "absolute" quantities. However, the emergence of
the resource theory of quantum reference frames as a new branch of quantum
information science in recent years has highlighted the need to identify the
physical conditions under which a quantum system can serve as a good reference.
Here we investigate the conditions under which, in quantum theory, an account
in terms of absolute quantities can provide a good approximation of relative
quantities. We find that this requires the reference system to be large in a
suitable sense.
