https://www.scientificamerican.com/article/redefining-the-kilogram/

The official object that defines the mass of a kilogram is a tiny, 139-year-old cylinder of platinum and iridium that resides in a triple-locked vault near Paris. Because it is so important, scientists almost never take it out; instead they use copies called working standards. But the last time they did inspect the real kilogram, they found it is roughly five parts in 100 million heavier than all the working standards, which have been leaving behind a few atoms of metal every time they are put on scales. This is one of the reasons the kilogram may soon be redefined not by a physical object but through calculations based on fundamental constants.

“This [shrinking] is the kind of thing that happens when you have an object that needs to be conserved in order to have a standard,” says Peter Mohr, a physicist at the National Institute of Standards and Technology (NIST), who serves on the committee that oversees the International System of Units (SI). “Fundamental constants, on the other hand, are not going to change over time.”