Smaller barrels also very often have much thicker wood proportionally than bigger barrels.. So that helps to counteract the surface area to volume ratio thing... And if you really want to get technicL about it etc... You could do things like seal every second stave of the barrel with appoxy or poly paint. Or glue on tinfoil... Probably lots of different ways to get around it and still work in smaller barrels.
i have a 40L barrel that I have done some oak aging in - its funky now, so a lambic/flanders type beer is what will go in it next. I have considered the one barrel version of a solera... Brew one batch at 40L and funk it up, then every six months or so, tap out a 19L keg's worth of the beer and top the barrel up with a fresh batch.
You get a similar sort of creeping age effect as you do with a multi barrel solera, just not to quite the same degree. For beer, you dont really want to end up with an average age of 7 years or whatever... You just want to be able to get a batch of oak aged lambic, averaging say 1-2 years old every six months, without having to maintain a bunch of small barrels to do so.
PS - my barrel is "old" so while its still letting out a little bit of oak, its more or less neutral and in future batches, I would be adding some chips for any oak character... The barrel is really for the sake of micro oxygenation, coolness factor and for funky beers, to play homestead for the bugs and critters.
