How is it not right? As guild level increases, the decay amount increases. Regardless of whether you are large or small.
As the guild member size increases, the decay also increases. But the more people you have, the more people there are to gain renown to offest that penalty.
The simple fact of this matter is that larger guilds are more likely to have slackers that do not get much if any renown from week to week. If they policed their members like the small power guilds they could rise to the guild level cap eventually as well.