When they step out of the wardrobe (or, rather, tumble: there is nothing graceful about their exit into the spare room) they find themselves more disoriented than they've ever been--which, after decades of training and war, and therefore head injuries, actually says quite a lot.
And it is not just that their bodies are smaller, but that they actually seem to work correctly. If they had spent less time in Narnia, perhaps they would have thought the opposite, would have been frustrated by untrained juvenile muscles, but even Lucy is--was--aged enough that her body had started to deteriorate, so the youthfulness is less frustrating than it could have been, though even that is not enough to make any of them at all glad to have left what had been their home for more than three decades.