As a "small" barrel user (have four 6 gallons and two 12 gallons), I can agree that, in the early stages, they aren't as productive as the later stages. In a 6, you can typically get all of the oak you need in about a month and therefore don't take advantage of all of the microx that you could. Second wine for two months, a little better. Third for three, better yet. Now that some of the barrels have reached the age that wine can sit for six months, much more benefit. In fact, in tasting, even the wines that get only a short sit are improved. Not to say that you can't do that with a flex and staves........
While the 50-60 gallon size is probably the "holy grail" where you can leave wines in new and once used barrels for a year or two, the practicality of producing that quantity of wine to fill one of those, at least for me, in my current home wine making situation, is impractical.
It may very well be that one day my production will change to a couple of big batches per year and use the big boy barrels, but thus far, have been very satisfied with the level of investment and return delivered by the small barrels, but hey, I'm just one dude with little barrels, YMMV..............
Wine is constant proof that God loves us and loves to see us happy. - Benjamin Franklin