  Nearby dwarf galaxies display a variety of effective radii (sizes) at a given
stellar mass, suggesting different evolution scenarios according to their final
"stellar" size. The TNG hydrodynamical simulations present a bimodality in the
z = 0 size - mass relation (SMRz0) of dwarf galaxies, at $r_{1/2,\star}$ ~ 450
pc. Using the TNG50 simulation, we explored the evolution of the most massive
progenitors of dwarf galaxies (z=0 $\log( M_\star / \mathrm{M}_\odot)$ between
8.4 and 9.2) that end up as central galaxies of their groups. We split these
dwarfs into three classes of the SMRz0: "Normals" from the central spine of the
main branch, and "Compacts" from the secondary branch as well as the lower
envelope of the main branch. Both classes of Compacts see their stellar sizes
decrease from z ~ 1 onwards in contrast to Normals, while the sizes of the gas
and dark matter (DM) components continue to increase (as for Normals). A
detailed analysis reveals that Compacts live in poorer environments, and thus
suffer fewer major mergers from z = 0.8 onwards, which otherwise would pump
angular momentum into the gas, allowing strong gas inflows, producing inner
star formation, and thus leading to the buildup of a stellar core. Compacts are
predicted to be rounder and to have bluer cores. Compact dwarfs of similar
sizes are observed in the GAMA survey, but the bimodality in size is less
evident and the most compact dwarfs tend to be passive rather than star
forming, as in TNG50. Our conclusions should therefore be confirmed with future
cosmological hydrodynamical simulations.
