The measure of activity in DDO is renown earned. The guild that earns the most renown per day should level the fastest. That is exactly what happens under the current decay formula.
Under the old decay formula, the larger a guild was, the more it was penalized (by reduced renown) and the less the renown that it's members earned counted. In fact the penalty was not even limited to the amount they earned! The old decay formula could (and did) even penalize them into losing more renown than they earned. Don't let the IRS hear about this one!