The original Yu-Gi-Oh! series remains a beloved anime nearly a decade and a half after its conclusion, and the trading card game continues to enjoy a great deal of popularity to this day. Many may believe that Yugi, the central protagonist, morphs into his alter ego, the Pharaoh, when a duel's about to begin, but that's a misconception -- outwardly, there seems to be very little change in-universe. In fact, the entire transformation sequence, as well as Yugi's altered height and voice, are all just visual cues for the audience to understand who's in control of the body.
There are a lot of hints that tell us that, while the viewers can understand who's Yugi and who's Yami Yugi, no one else can (that is, until Shadi makes his entrance). The only person who notices the change is Kaiba during his first duel with Yugi, who mutters, "What the --?" after Yugi cedes control over his mind to the Pharaoh, and it might be because of the ancient history that they share. But for the rest of the arc, the fact that Yugi morphs into the Pharaoh is all but unnoticeable to onlookers. Even the holder of the Millenium Eye, Pegasus, is unable to determine the existence of a second Yugi during their initial Shadow Duel, and Yugi's friends aren't able to discern the difference between the two.
Yugi and Yami, for the most part, simply don't interact at all until the former stops the latter from leading Kaiba to his death during the Duelist Kingdom arc. We're told of his various names during the conclusion of said arc, and otherwise, they're treated as the same person by those around them.
The manga also similarly implies that Yugi's physical appearance never actually changes when the Pharaoh takes over. When Yugi changes into Yami to face off against the escaped prisoner Jiro, who holds Tea hostage in a restaurant, the crowd surrounding them sees no difference between the two. Tea's the only one to realize that Yugi wasn't the one to save her from the situation as he has no memory of it. You'd imagine that someone would comment on the change or even that Jiro would notice, but considering that none of them do, it seems safe to assume that Yami remains at Yugi's (rather short) height.
It's only when Shadi arrives at the conclusion of the Duelist Kingdom arc that the differences between Yugi and the Pharaoh start to really get explored. To Shadi, Yugi and Yami look identical. But Yugi's mind is that of a child's, while Yami's is an apparent labyrinth full of traps that Shadi nearly falls prey to. Indeed, only the holders of the Millenium Items can recognize the difference between Yami and Yugi. As for physical differences, it seems that only the anime viewers are able to perceive them.