We provide an experimental investigation on the mechanism of the bubble formation from sub-millimeter orifices under variable gas flow conditions VGFC. The effect of influential parameters including the volumetric gas flow rate Q, the orifice diameter d_or, and the volume of the gas reservoir V_c upstream of the orifice are investigated in detail. We found that, by enlarging V_c,q increases. This affects various aspects of bubble formation dynamics such as the bubble base expansion, the apparent contact angle at the three-phase contact point, and the bubble detachment criterion. The effect of V_c is best presented by its dimensionless group, the capacitance number Nc. The minimum Nc from which the influence of the gas reservoir becomes important depends on the orifice size. Moreover, a significant increase in V_c leads to a change in the bubbling regime towards multiple bubble formation.