  Light localization due to random imperfections in periodic media is paramount
in photonics research. The group index is known to be a key parameter for
localization near photonic band edges, since small group velocities reinforce
light interaction with imperfections. Here, we show that the size of the
smallest localized mode that is formed at the band edge of a one-dimensional
periodic medium is driven instead by the effective photon mass, i.e. the
flatness of the dispersion curve. Our theoretical prediction is supported by
numerical simulations, which reveal that photonic-crystal waveguides can
exhibit surprisingly small localized modes, much smaller than those observed in
Bragg stacks thanks to their larger effective photon mass. This possibility is
demonstrated experimentally with a photonic-crystal waveguide fabricated
without any intentional disorder, for which near-field measurements allow us to
distinctly observe a wavelength-scale localized mode despite the smallness
($\sim 1/1000$ of a wavelength) of the fabrication imperfections.
