More the cog spacing than the size. as they've got closer, when you have an older derailleur working on a long actuation ratio, a small adjustment of the cable can have a relatively big effect on the derailleur tuning. Dirt & shit adding friction to the cable accentuates the problem.
Shortening the actuation ratio (which SRAM did many years ago) makes the derailleur less sensitive to fine cable adjustments, enabling better fine-tuning because you need to adjust more cable to get the same effect, and also less sensitive to cable friction, up to a point. The trade-off is a longer shifter stroke, although that can be altered through internal design of the shifter.
The funny thing is, when SRAM went 10-sp, they lengthened their actuation ratio (and carried it over to road 11-sp.), but went back to damn near what they did originally when they went 11-sp. MTB. I don't know what they've done to it for 12-sp.