This paper addresses the issue of scale invariance in evaluation protocols for Salient Object Detection (SOD). Specifically, we focus on scenarios where multiple salient objects of significantly different sizes appear within a single image. We demonstrate the size sensitivity of existing SOD metrics and theoretically demonstrate that the evaluation results can be decomposed in such a way that the contribution of each component is directly proportional to the size of the corresponding region. To address the problem of underestimation of small objects due to this size imbalance, we propose a Size-Invariant Evaluation (SIEva) framework that evaluates each component individually and aggregates the results. Furthermore, we develop a model-agnostic optimization framework (SIOpt) that adheres to the principle of scale invariance and improves the detection of salient objects of various sizes. Furthermore, we present a generalization analysis of the SOD methodology and provide evidence supporting the validity of the new evaluation protocol.