So let's compare 3 guilds and see how they get treated by the current system, a 6 man guild, a 50 man, and a 150 man. I'm going to use a level 70 guild as my example, but the raw numbers are sort of irrelevant, its more the ratio of what is going on with renown that is important.
So first, lets start buy seeing how much renown each member would have to pull to go from level 70 to 71. It would require 17, 895, 550 renown to achieve.
6 person - 17895550/6 = 2,9825,250 per person. This guild size would get a 300% renown bonus so to figure the base renown each person would have to pull to level /4 = 745,648 per person
50 person - 17895550/50 = 357911 per person to level
150 - 17895550/150 = 110304 per person to level
So clearly, in terms of how much each person needs to pull, even with small guild bonuses applied, larger guilds have an advantage here and could level by doing a fraction of the work.
Now lets look at decay. Assuming spoonwelders post in the decay discussion thread is correct, and all level 70 guilds have 10805 decay per day, lets examine how that impacts guilds.
6 man - 10805/6/4 = 405 base renown per player per day to beat decay after the 300% bonus is factored out
50 man - 10805/50 = 216 per player per day
150 man - 10805/150 = 72 per player per day
Again, bigger has a massive advantage.
Under the older system people complained about being unfair larger guilds had an advantage with being able to pull less renown per person per level, and smaller guilds had an advantage with being able to pull less renown per person per day to overcome decay thanks to small guild bonuses. Now the scales are tipped entirely in favor of one guild size and one guild size only, which happens to be as many people as you can possible recruit.
Please turbine fix this skewed, broken, and biased decay system. I'd prefer for decay to actually mean something to all guilds again rather then just small guilds, but if it is the intent of the devs to make decay irrelevant and let every guild waltz their way up to 100 then small guild bonuses should all be multiplied by 10x so small guilds don't have to pull such a ridiculous amount of trophies more per player per level AND be expected to deal with a heavier amount of decay per player then large guilds. Basically nobody gets an easy button to a level 100 guild(which admittedly is my preferred method) or give every guild regardless of their size an easy button so we can all feel special with our level 100 guilds.