Maybe the limit is the number of maintainers? One person can only deal with so many people so that's the natural limit for how many maintainers you want. Each maintainer sends one merge request during the merge window and two or three after the merge window. Except for "tip" which gets merged 50 times.
It would probably help ARM to have a single maintainer. Aren't we supposed to be unifying ARM so that a single kernel can boot on everything? One solution to the config file problem would be to add an option to git to clean up the logs. "git log -p --no-merges --no-arm-config-spam v2.6.34.." The config spam far outweighs the merge spam so this would be useful. ;)