We simulate a binary soft disk system 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 fragility is not correlated with structural ordering, but rather with the spatial character of heterogeneous dynamics.