From a mechanical perspective, larger solder joints are generally preferred when assembling package-on-package (PoP) components. Just as a large set of gears can handle more power, fortified interconnects add a measure of reliability to BGAs and CSPs. That is why in many cases, dipping paste is used instead of PoP flux for package-on-package stacking and BGA rework - to increase solder joint volume. The added volume of solder helps keep the solder spheres in contact with interconnect pads throughout the reflow cycle, combating the effects of warpage. This will help you increase the solder reliability and the final yield of your PoP assemblies.
Here are some links to learn more about the PoP solder paste process:
Package-on-Package Placement (3/5)