I do wish all of it had been as big as that first volume Viz printed, but it made it more expensive. The size change, and the cover designs, are both designed to lure in more readers. I say I'm willing to have an unwieldy shelf if it gets even one more person to read Phoenix than otherwise would.
That said, I do prefer the US release to the French, which is even smaller, on paper which browns easily, and in very thick volumes which are hard to hold given how small they are. Once the US set is complete then, other than the fact that French vol. 5 contains Robe of Feathers, there will be no reason to choose the French over the superior English - that's rare for Tezuka releases, which are usually better in French. The French covers are a bit more distinctive in telling you which story is in which volume, but they're just not as rich or colorful, not enough to make up for the smaller size: