They key difference is that an attribute can only appear once on an element, and consequently only have one of its variants present. Classes can present multiple variants at once.
At a surface level `<div class="Card" data-size="big">` isn't really any better `<div class="Card big">`. But `<div class="Card" data-size="big" data-size="small">` is not valid HTML - the second attribute is discarded. While `<div class="Card big small">` has no such contract, and as such is valid HTML and the CSS is unlikely to account for it, perhaps doing surprising things.
At a surface level `<div class="Card" data-size="big">` isn't really any better `<div class="Card big">`. But `<div class="Card" data-size="big" data-size="small">` is not valid HTML - the second attribute is discarded. While `<div class="Card big small">` has no such contract, and as such is valid HTML and the CSS is unlikely to account for it, perhaps doing surprising things.