Agree with responses above. In particular, bearing capacity for a larger diameter shaft increases the B*gamma/2 term proportionally (but not the depth or cohesion terms), so the bearing capacity of a large diameter shaft in sands would be higher. On the other hand, if they consider settlement response of the tip, allowable bearing capacity for a large diameter shaft may be much lower since mobilization of bearing pressure may be limited by the overall settlement (BC mobilized at 10 - 15% of diameter). If for shafts roughly the same diameter as the helical pier, then may be OK.
As noted above, the geotechnical recommendation could be uniformly conservative, since the geotech may have a high factor of safety on their tip resistance. Low allowable bearing might be presented in part because if stronger and weaker portions of the deposit are available (variable non-homogeneous strength) one can't rely on the tip bearing on the strongest layer (either because one can't know exactly which depths the highest strength is, or one can't rely on the contractor not to overdrill, whereas you can advance the helical until torque is reached), or in particular they may have assumed cleanout of the shaft tip may not be perfect, particularly for small-diameter shafts.
