Most SRBs are long and skinny, probably because most are used low in Earth's atmosphere where aerodynamic drag is large.
But here are some very short SRBs, one seemingly spherical except for the nozzle at one end, from Spaceflight Now's NASA narrows design for rocket to launch samples off of Mars
I wonder if the preferred shape of SRB's would always be closer to spheres than to pencils if it weren't for Earth's atmosphere, or if these are short and squat of necessity due to their need to be packaged and sent to Mars first.
Considering exposed area would increase as $r^2$ for a sphere compared to $r$ for a cylinder, do they have more strongly peaked thrust curves?
This diagram illustrates one concept for a two-stage, solid-fueled Mars Ascent Vehicle. Credit: NASA/MSFC
This illustration shows a concept of how the NASA Mars Ascent Vehicle, carrying tubes containing rock and soil samples, could be launched from the surface of Mars in one step of the Mars sample return mission. Credit: NASA/JPL-Catlech