Vacancy-mediated transport drives the functionality of oxide-based nonvolatile memristive devices. Here, we report the size dependence of TiO2/Nb:STO heterojunctions for electroforming and the subsequent resistive switching process. Conductive AFM measurements suggest that the forming and reset voltages both decrease with increasing junction size. We also show oxygen flow ratio changes during fabrication, and post-annealing impacts the set voltage and resistance ratio through changes in available oxygen vacancies. Finally, a polarity reversal between eight-wise and counter-eight-wise switching occurs after vacuum and ambient anneals, thus modulating oxygen vacancy availability and changing (reversibly) the mechanism from vacancy migration to an electron trap/detrap process.