ALL Guilds over 10 accounts got a reduction in decay from old to new. 10 and under did not. Even though 10 and under did not, they have the highest size bonus and are still able to overcome decay as shown.
If history is a guide and Turbine decides to drop the account numbers down to 10 ( and I personally doubt that will be the solution) I fear it would come hand in hand with a very small size bonus reduction.
Maybe this is an incentive for the very small to be inclusive? Maybe Turbine wants people in Guilds with lots of people? Maybe the 10 and under account are such a small percentage of the total guild population, they do not see it as an issue because they are advancing. Maybe the very small guilds are reaching the level ranges Turbine thinks the average 'should' attain as not all guilds will reach top level.