Unless something else is going on, Adobe Illustrator's Optical Kerning seems to scale kerning results differently according to the point size.
Briefly, a client asked me to modify kerning* in a font so that Adobe's Auto Kerning matches Adobe's Optical kerning (applied to figures only). It is my client's client that whats this. For what ever reason, They insist on working with Adobe Optical kerning, but the client's system is Kind of like a BIM/CAD system, (but not for buildings) that has to convert Illustrator files to SVG to function and therefore has no access to Optical kerning.
For type set above 72 points, my adjustments are as accurate they can be... (The native kerning being limited to units on a 1000 x 1000 grid while the Adobe drawing space is, of course, capable of much finer increments... Meaning that most pairs involve rounding up or down by a unit to get a result that can be off by up to half a unit in FontLab but for practical purposes, perfect. Below 72 points points, accuracy starts going down, becoming increasingly noticeable below 60 points.
The only reason I can think of for Optical Kerning making proportionally larger adjustments at smaller sizes would be to mitigate the effect of crowding text at smaller sizes.
At smaller sizes adding tracking in Illustrator does fix the problem:
- 72 point and up, all is good.
- @ 60 points, add 3 units tracking = no discernible difference up to 10 characters
- @ 36 points, add 10 units tracking = no discernible difference up to 10 characters
- @ 27 points, add 13 units tracking = no discernible difference up to 10 characters
- @ 21 points, add 15 units tracking = no discernible difference up to 10 characters
* It is allowed in the EULA