On all hubs, but since mtb hubs see much higher and frequent impact loads, bigger bearings are more beneficial on them than road hubs. I think if dt swiss use 6802 on dt180 hubs, then they should be fine for most road riders.
Bigger bearings would theoretically have more drag as they have more balls, but that means the loads are distributed over a larger contact area, meaning the loads on individual balls are lower, meaning they will wear slower. But the larger contact area means there will be marginally more drag, virtually irrelevant amounts for bike applications though. Seals make magnitudes more impact on bearing drag.
The tradeoffs with bearing sizes in hubs are usually size and weight vs durability:
• smaller bearings means lower weight and smaller, fractionally more aero hub
• bigger bearings means better durability against wear and dirt ingress.