Qitzur Shulchan Arukh – 62:9
One must measure and weigh with a generous eye, so that there will be a little more beyond the measure. As it says “a full and just measure you should have”. The Torah says, “be just at your own expense, and give to him”.
As we already established erring in a measurement in one’s own favor is particularly odious. The Torah considers it so bad that owning the tools for it, without even using the dishonest measures, is enough for Hashem to brand the owner a “to’eiavah to Hashem”!
Since it’s impossible to measure product without erring, and since erring in one direction is so wrong, we are advised to “play safe” and overestimate a little in the counterparty’s favor.