Morgana doing what she does best - showing off and being a huge, insufferably smug snake-balloon.
Like many snakes, Morgana's jaws and stomach are highly elastic. When combined with her magic, this allows the serpentine sorceress to expand her body with air, liquid, or just about anything else she desires (or can fit into her mouth), turning her into a huge, scaly balloon. Once sufficiently distended, the nagini can use her inflated curves/bulk to pin, smother, or otherwise bully rivals and lovers alike.
To the surprise of no-one, Morgana is capable of swallowing even large humanoids whole. She typically does this to punish or torment her victims, rather than out of actual hunger. She can suspend her digestion when desired, and her 'prey' is protected from harm while inside her, making it a convenient way to conceal someone (or something) from detection. Of course, the nagini isn't the only creature in the world who can expand, nor is she indestructible, and more than once Morgana has found herself in grave distress after recklessly swallowing someone she cannot safely contain.
