  Quantum experiments with nanomechanical oscillators are regarded as a testbed
for hypothetical modifications of the Schr\"{o}dinger equation, which predict a
breakdown of the superposition principle and induce classical behavior at the
macro-scale. It is generally believed that the sensitivity to these
unconventional effects grows with the mass of the mechanical quantum system.
  Here we show that the opposite is the case for optomechanical systems in the
presence of generic noise sources, such as thermal and measurement noise. We
determine conditions for distinguishing these decoherence processes from
possible collapse-induced decoherence in continuous optomechanical force
measurements.
