Larian is having trouble fitting Baldur’s Gate III on the Xbox Series S, the lower-priced and lower-powered console in Microsoft’s ninth-generation lineup.
I was looking up more information on why there’s such an issue getting BG3 on Xbox, and found this article with a lot more detail on the topic.
EDIT: The issue isn’t graphics or frame rate; it’s memory. The article goes into detail.
Also keep in mind that PC doesn’t have unified memory. So there’s usually at least 8GB of VRAM in addition to whatever amount of main memory you have.