Small bearings are smaller overall. So, they can handle less string layers. They spin faster, because of some random physics. Because of the smaller gap, they decrease vibe, and decrease gyroscopic precession, so they don’t turn around while you are yoyoing. Because of the small gap, they can still provide tight binds when the thing is spinning slowly. They can however, give less sleep times, because the small gap can make it easier for the string to rub against the response.
Big bearings are bigger overall. So they can handle more string layers. Because of the wider gap, there is a higher chance of vibe, and gyroscopic precession. Because of the big gap, it can be more unresponsive. It can also give better sleep times, because of the wide gap, making it easier to prevent the string from rubbing against the edges of the yoyo.
EDIT: I’m not sure which one should have a bigger RPM actually ???
EDIT: Yup, smaller should spin better, thanks JM!