what impact is being measured when we use the effect size measure. Since it is only used when a null hypothesis is rejected, we are not focusing on...

what impact is being measured when we use the effect size measure.

Since it is only used when a null hypothesis is rejected, we are not focusing on the differences in measures.  We are looking at the "cause" of our rejecting the null - is it due to a large sample (small effect size value) or is it due to a practical difference/interaction in the variables (large value)?