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Orbital ATK Pegasus: 16.9 m × 1.27 m
RocketLab Electron: 17 m × 1.2 m
Apart from that, the two rockets are very different: the Pegasus is a solid-fuel, air-launched rocket with a mass of 18.5 t and can bring 440 kg to LEO, while the liquid-fueled Electron weighs in at only 10.5 tons and gets ≈200 kg to LEO.
Is there more to this? Has RocketLab perhaps voiced any intent of air-launching the Electron at some point?
The design constraints for air launch are more about the first stage being able to light while horizontal (no fuel sloshing away from intakes), and the structural additions to support being hung fully laden sideways rather than vertically in the same direction as thrust loads. There also needs to be the flight controls to achieve the pitch up, which may involve managing asymmetry in drag and/or thrust vector.
So the things to look for would be design changes to hold the rocket from the side rather than resting on the engine frame pre launch, design to allow motor operation while sideways, aerodynamic control surfaces and changes to the control system to make it more autonomous (air launch will involve release, then motor start so final checkout and ignition must came from within rocket itself).
Do not have any specific sources but would suggest the diameter/height is driven be drag/structural engineering needs that make the 'caliber' of rockets reasonably consistent for a given capacity.
+1 You might want to add that the LOX tank has a large area/volume ratio and not much room for insulation, so the boil-off rate might be high, then mention how long it takes to get to launch hight, how to feed make-up LOX from the plane, etc.
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