I’m no expert on fit (thus the fit threads) and the myriad jeans fit descriptions (mom, dad, boyfriend, traditional, slim, skinny, etc) is beyond me! The way I think about fit is (1) slim fit - which is close, hugging, body con (2) loose (or traditional) fit - skims the body with some ease, but follows the contours of the body underneath (3) oversized fit - generous proportions with little attention paid to proportions or contours of the body. These fits can come in all fabrics and fabrications - unstructured knits and tailored woven garments. They can be worn by all body types as well.
Sizing up or down, or choosing petite, regular, or tall should mean that a garment’s proportions all,change, however manufacturing standards aren’t necessarily uniform. A ‘tall’ size trouser should not only be longer in leg length, but longer through the rise, and as such have a proportionately longer zipper - but this is not always the case.
Specifically to your question regarding oversized garments, my opinion is that some garments are designed specifically to be worn oversized and as such there will be volume in some parts, but proportion in others. In other words, there might be a dropped shoulder, and a wide body, but the sleeves and torso aren’t overlong. (How well an oversized garment looks depends on styling and your like (or dislike) of the fashion.) I think ‘sizing up’ can work - but probably more so when the wearer is doing the sizing up to accommodate some disproportionate aspect of their physique. For example, sizing up to get a good fit through shoulders and arms might result in an oversized ‘look’ if the torso and bust is modest by comparison.
The ‘success’ of oversized garments as a style conceit is in creating something visually interesting/appealing. Usually that means hinting at the body underneath, either by collapsing against it, or revealing a little skin (sliding off a shoulder, etc.) It is my opinion that a lot of oversized garments get purchased for the cozy factor or for hiding some perceived figure flaw, rather than for style reasons. I think wearing oversized garments in a stylish/attractive way takes a lot of work!
