  Decades of work on beam deformation on reflection, and especially on lateral
shifts, have spread the idea that a reflected beam is larger than the incident
beam. However, when the right conditions are met, a beam reflected by a
multilayered resonant structure can be 10\% narrower than the incoming beam.
Such an easily measurable change occurs on a very narrow angular range close to
a resonance, which can be leveraged to improve the resolution of sensors based
on the detection of surface plasmon resonances by a factor three. We provide
theoretical tools to deal with this effect, and a thorough physical discussion
that leads to expect similar phenomenon to occur for temporal wavepackets and
in other domains of physics.
