Just like to add the following to the clarify the above;
In tension the failure of the material is limited by rupture occurring when the Ultimate Tensile Allowable for the material (ftu) is exceeded; but note Aluminium is ductile so exhibits a reasonable amount of elastic deformation before failure/rupture; infact the failure does not occur until permanent deformation occurs, so for single event Ultimate cases allowing for the permanent deformation (plastic correction) can allow you to exceed the "elastic" allowable. So in tension the material property is the determining factor.
In compression the failure mode is a factor of the Fcy (Allowable Compressive yield stress of the material) and the geometry (normally the aspect ratio / thickness width ratio part as PDR stated). For thin / high aspect ratio sections a material will initially become locally unstable and buckle, however a degree of elastic buckling may be permitted (ie: the point at which it will spring back to the unloaded shape). Beyond the local instability you get the crippling phase where the material loses any capability to carry load.
The geometric t/width ratio correction means the local instability and / or allowable crippling stress can be significantly below the actual material compressive yield allowable. The allowable compressive stress of a most aviation grade aluminiums (2000 or 7000 series) is in the region of 70 to 80% of the Ultimate Tension Allowable.
In turn the geometric correction means the instability / crippling compression allowable is only a proportion of the compressive allowable.
So the tension capability of a "thin geometric" shape is significantly greater than the compression capability.
Just trying to think PDR, while the normal level flight 1 g case load case is upper compression, lower tension it isn't a critical case; the negative gust cases could easily see the load reversal meaning upper and lower booms probably need to have mirrored dimensions for the ultimate cases. The 1 g level flight case is more an issue for fatigue; the zero to tension is more damaging than zero to compression.
As the human being can take more positive g (6 to 9) than negative g (-3g); for an aerobatic aircraft pulling higher positive sustained g manoeuvres then sustained negative, it would means you would generally need the upper boom to be made a lot stronger than the lower boom.