This is reflected in a recently updated support document, as spotted by MacRumors. Apple says the iPhone 12 mini can achieve its 12W charge speeds via MagSafe with a USB-C charging adapter that is at 9V/2.03A or higher. The others get their 15W power delivery via charging adapters that are at 9V/2.22A, 9V/2.556A or higher.
The support document also notes that if there’s any accessories plugged into the Lightning port, charging slows down even further to 7.5W. Apple cites regulatory standards as the reason for this reduced charging speed. But the company doesn’t say why the iPhone mini 12 has a lower charging speed via MagSafe compared to its bigger siblings. On one hand, it’s a small difference in speed, and is unlikely to sway purchase decisions. The small size of the iPhone 12 mini would definitely be a bigger deciding factor. But at the same time, it’s better to know that it’s a thing now before they get to people’s hands. (Source: Apple via MacRumors)