S. x Doreen's Colossus came to me about 25 years ago with a label S. (flava x oreophila) and the appearance of this cultivar supports its parentage.  This plant is consistently the largest and most massive that I grow in my greenhouses.  The rhizomes are very large and densely branched.  In my experience, no other Sarracenia fills and breaks out of pots as quickly as S. x Doreen;s Colossus.