No, it could get a little smaller, it could get a little bigger. (There's more to it than this but oh well...) But the effect size isn't magically going to get larger just because you take a larger sample size. That doesn't make sense. The result might become more significant (smaller p-value) because you have a better estimate of the error and the standard error of the difference will decrease with a larger sample size. But that doesn't mean the estimate effect itself will get larger.2. If a study shows that Group A mean is 5% greater than Group B, does it mean if i have bigger sample size Group A mean will only get bigger? Or it can also go in the reverse direction (where Group B mean gets bigger)?
Ultimately with a larger sample size you're getting a better estimate of what the true means actually are.