It was ArtForz or someone associated with one such I recall said they would only scale up to no more than 1/2 of total hash rate because beyond that they are competing with themselves and experience diminishing returns.
I would be interested to see their reasoning on that, my calculations put the sweet spot at 20%, not 50%. In my calculations adding hash power not only devalues your existing investment, but increases the difficulty within 2 weeks. Unless your hardware is both cheaper and more energy efficient than everybody else's, it makes very little sense to expand beyond 1/5 of the mining pool.