@NOT THE BEEEEES! The way the met gala works is that people usually get invited by fashion houses, who spend several hundred thousand pounds on reserving tables. The fashion houses usually then invite famous or interesting people to sit on the table, often with the pretext of having them wear their clothes - some celebrities will get couture pieces made with the fashion houses, but sometimes they're literally given clothes to wear with minimal input, on the basis that the fashion house wants their clothes photographed to add hype.
It's possible that Anna Wintour invited Ellen Page directly (this does happen with some celebrities) but it's more likely than Ellen was invited by Balenciaga and then dressed by them. The suit she was wearing was a Balenciaga tuxedo:
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which was paired with black Balenciaga Triple S sneakers.
Balenciaga is currently experimenting with oversized tailoring (to contrast with how suits have been skinny fit and extremely tailored for a while). Regardless of your opinion on that style, a general rule of thumb is you want some balance of oversized and regular pieces to add contrast, and to focus on how it's oversized (i.e. ensuring it isn't oversized in key areas like around the shoulders).
What they did here is just throw a high concept oversized tuxedo onto Ellen Page, without thinking of how it would literally make her look like a kid wearing her Dad's suit because she's tiny. I think they probably did tailor the suit a bit (it seems to fit around the shoulders and the trouser length seems ok) but it's just the cut itself that makes her look so bad (along with the choice of putting her in sneakers instead of dress shoes).