The answer is largely that, while this is conventional wisdom, none of the rationalizations put forth for it hold up to even casual scrutiny. The problem with received wisdom like this is that it's effectively impossible to overturn: once an idea gets into your head about how something "ought" to be perceived it's really difficult to get rid of (hence the studies where expert wine reviewers describe white wine tinted red with red wine flavor descriptors, despite not actually having any of them). So you can get some pattern by random chance that then gets upheld because your brain "knows" it's true and makes it so (or you just expect it going in, so many people "know" that larger formats age differently that a neutral observer would expect them to confirm it). It's also possible that these things used to be true but with modern storage technologies and QC they're not.
I tend to start from the idea that unless there's a good reason for something to affect how a beer (or wine) ages, it probably doesn't matter. I've never seen any reason for size variations that's even close to good, so it almost certainly doesn't matter.
ETA: What happened to the modified stupac'd image? I wanted to stupac it by saying he should've used a picture of my lambic racks.