If a typeface is well spaced for use at paragraph sizes, the white inside and between letters should appear — generally speaking — equal. If one goes about spacing the same face for display use, you’ll need to space it somewhat tighter. If you decrease the white between letters, shouldn’t you also decrease the white inside letters? This will most likely be achieved with a display variant. My question is: How do you decrease the internal white? Draw the letters narrower? Where do you take it from? Increase the size of serifs? Lower the x-height? Make the stems heavier?
I wonder, since typically, a display variant would be lighter, have a taller x-height and even sometimes smaller serifs: all increasing the white inside letters, leaving the heavy lifting to width.