Not sure if I understand fully about your question but here are some thoughts.

Electrode diameter to some extent determines the current that can be placed... Meaning a small diameter electrode carries less current than a large one yes?

So a small diameter electrode may not produce enough current to make a sound weld on a massive base metal thickness.  A small diameter electrode might not be able to produce (easily) the correct size fillet when base metal thickness dictates a large sized fillet.

Economics/efficiency are also an issue.  A large electrode will deposit more metal.

However, I'm not aware of a code specification that limits an SMAW electrode size for a particular base metal thickness........ But there are some electrode size limitations for welding out of position (vertical and overhead) due to the fact that it is more difficult to make those welds with very large electrodes.  Manufacturers do have suggested relationships for electrode size and base metal thickness.

Say more about your particular issue.

Weld deposition rates vary directly in relation to the current density used. In SMAW, the smaller diameters are used with low currents for the joinng thin sections whereas the larger electrodes require higher current, which produce higher weld deposition rates.