This is the only MOTU show where Adam is the same age/build as He-Man; all other shows since have shown Adam as younger than He-Man. While Filmation's trademark of Limited Animation is part of this, keep in mind that Clark Kenting is at work here: a simple change in clothing and attitude works pretty well on Eternia, a fantasy world filled with wonders where no one would suspect that a great hero would be a young prince.
This is why Queen Marlena would know Adam's secret. As an outsider from Earth, she could see through the disguise easily where natives wouldn't, and because she's from Earth where secret identities are a commonplace theory (whether existing or fictional).
Adam keeps up the image of a lazy, irresponsible fop despite being just as brave and mature as He-Man not only to protect his secret identity, but also because it's the only moments of peace he actually gets where he can indulge himself in what he likes doing (fishing, painting, etc.) Notice that Man-at-Arms (one of the few who know Adam is He-Man) almost always defends Adam's antics as a deserved release from the massive responsibility he was born to.
Further helping the disguise is the fact that most male characters have the same body type as He-Man. Thus the familiar appearance is further downplayed.
Cringer is a sentient being and unashamed coward who hates being transformed into Battle Cat and yet his alleged best friend regularly does so without permission or apology to use him as a riding mount. That's an abusive relationship if ever there was one.
More Fridge Sadness, but Orko's backstory in this cartoon is pretty tragic when you stop and think about it (or at least the most coherent version. On his own world, Orko was a powerful and respected wizard, but when he got swept up in a cosmic storm, he found himself stranded on an alien world. Then, trying to save a young Prince Adam from dying in a tar pit, he loses the amulet that helps control and focus his powers, which combined with the very different laws in physics, renders him cursed with Wild Magic that he can't control. Whilst the royal court takes him in, he's gone from an archmage to a glorified court jester, with everyone treating him as a bungling magically inept buffoon because his magic no longer works for him. Talk about a downgrade! No wonder this became the official backstory for Orko (substituting a wand for his amulet and making it important for him even on Trolla) in the Darker and Edgier 2002 reboot.
