  We simulate binary soft disk systems in two dimensions, and investigate how
the dynamics slow as the area fraction is increased toward the glass
transition. The "fragility" quantifies how sensitively the relaxation time
scale depends on the area fraction, and the fragility strongly depends on the
composition of the mixture. We confirm prior results for mixtures of particles
with similar sizes, where the ability to form small crystalline regions
correlates with fragility. However, for mixtures with particle size ratios
above 1.4, we find that the fragility is not correlated with structural
ordering, but rather with the spatial distribution of large particles. The
large particles have slower motion than the small particles, and act as
confining "walls" which slow the motion of nearby small particles. The
rearrangement of these confining structures governs the lifetime of dynamical
heterogeneity, that is, how long local regions exhibit anomalously fast or slow
behavior. The strength of the confinement effect is correlated with the
fragility and also influences the aging behavior of glassy systems.
