This is gonna be a weird one. Is it terribly bad if some capitals get wider in the Regular weight than they were in Black? I mean the advance width – it found it growing a bit more as more spacing was needed for the lighter weight, so I kept tweaking the original Black making it wider, so as to keep things more or less "logical", but now I've started to wonder...
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So the reason why lighter weights have larger side bearings is that the size of the bearing is related to the negative space in the letters (and the shape of the glyph and other things too), the lighter the letter the larger the side bearings are.
It is not only how caps are spaced but lowercase too. Although there are few exceptions, check the link, I actually just watched this video about spacing yesterday : How to Space a Font. FontLab Studio 5 tutorial with Thomas Phinne