Bearings are like building blocks. We have many size ball bearings which have the same bore size. As the cross section of the ball bearing gets larger the bearing can handle heavier loads, with slower speed limits than the thinner bearings. Bearings can also have common OD sizes. Again, the bearings with the larger cross-sections will handle the heavier loads and slower speeds. Bearings can have common OD, bores and widths across bearing types.