Multiple mechanisms of low salinity waterflooding (LSW) orchestrate the increase of oil recovery. One proposed mechanism is surface roughening which alters the wettability in pores. However, clay swelling that generates surface roughening also simultaneously reduces the pore size. Therefore, we study the implication of surface roughening and pore size of the rock that leads to oil recovery in LSW at different brine compositions and clay contents. We measure the incremental oil recovery due to brine dilution sequences by using a modified-Amott imbibition cell. The results suggest that oil recovery due to brine dilution is minimal when the cores have clay content above 20%. The pore size alteration effects compete with surface roughening. We find that the presence of divalent ions hinders the clay swelling. This implies a smaller pore size alteration and the least amount of surface roughening. In addition, the evolutions of deduced pore size distribution through T2 NMR confirm the results of LSW imbibition. Further, zeta potential measurements on the sandstone suspension provide the surface-charge to understand the oil recovery mechanism. This research suggests that the pore size alteration and surface roughening are important mechanisms for incremental oil recovery in sandstone during LSW operation. These findings might aid the rational design of LSW in the future.