Existing literature reports inconsistent trends in the peak impact load (increasing, decreasing, or non-monotonic) with plate size during flat-plate water entry. A prevailing view has long held that smaller plates experience higher peak loads due to reduced air entrapment, which weakens the buffering effect, an interpretation that remains qualitative and unverified. In this paper, the influence of plate size on peak impact load and the air cushioning dynamics are numerically investigated using both 2D and 3D models. Contrary to conventional expectations, we find that as plate size decreases, the entrapped air volume (both total and per unit impact area) reduces, yet the peak impact load, quantified by maximum slamming and pressure coefficients, also decreases monotonically. To elucidate the underlying mechanism, the compression dynamics of air layer, which physically govern the peak load, are analyzed via a control-volume framework bounded by the plate bottom and the free surface underneath. The analysis reveals that the air compression is dictated by two competing processes: contraction of the control volume and air outflow through the control surface (i.e., peripheral gaps). The former promotes air compression, whereas the latter mitigates it. As plate size decreases, both processes intensify; critically, the markedly enhanced air escape dominates, thereby attenuating the net compression of air layer and leading to a monotonic reduction in the peak impact load.