It looks like there was a change to font rendering on macOS between the current stable ST3 build and dev 3161 (though this could have happened on an earlier dev version).
Top: stable, bottom: dev (font: Menlo, size: 14)
Comparing the above to other apps (e.g. vi in Terminal), the top (stable) rendering appears to be the “correct” rendering/letter spacing.
Tested on macOS 10.13.4.
