  Variations of transport efficiency in structured environments between
distinct individuals in actively self-propelled systems is both hard to study
and poorly understood. Here, we study the transport of a non-tumbling {\ecoli}
strain, an active-matter archetype with intrinsic size variation but fairly
uniform speed, through a periodic pillar array. We show that long-term
transport switches from a trapping dominated state for shorter cells to a much
more dispersive state for longer cells above a critical bacterial size set by
the pillar array geometry. Using a combination of experiments and modeling, we
show that this anomalous size-dependence arises from an enhancement of the
escape rate from trapping for longer cells caused by nearby pillars. Our
results show that geometric effects can lead to size being a sensitive tuning
knob for transport in structured environments, with implications in general for
active matter systems and, in particular, for the morphological adaptation of
bacteria to structured habitats, spatial structuring of communities and for
anti-biofouling materials design.
