Large-bubbles and small-bubbles represent two competing dominant structures in bubble columns. Understanding the structure evolution is of great significance in physical modeling. Dynamic gas disengagement (DGD) and dual conductivity probe (DCP) methods were applied to demarcate large and small-bubbles. While the number-based BSD only presents a single peak, the dual peaks could only be found from the volume-based BSD. The demarcation method based on the holdup ratio of the two bubble groups proves more reasonable to reflect the intrinsic nature of bubble structure. A sharp change of gas holdup was detected for the uniform distributor. The difference in gas holdup structure of coarse and uniform distributors was contributed mainly by small-bubbles at homogeneous regime, yet by large-bubbles at transition and heterogeneous regimes. However, the coexistence of small and large-bubbles is the intrinsic nature, and the effect of gas distributor is marginal in heterogeneous regime.