The instantaneous velocity at the flange on the discharge side of a centrifugal pump is going to be higher than on the suction, because of the action of the impeller. So the discharge flange only needs to be a fraction of the suction size to pass the same flowrate. Further, large diameter suction flanges mitigate against cavitation.

The suction side is made bigger not to restrict the flow.

You may fit an eccentric on the discharge side if you want to or when the terrain requires it. No harm can be done. You may even install it flat side down.

The most improtant issue in sizing the pipes for discharge and suction is the flow velocity. Generally speaking, you need much slower flow than the discharge. This due to:

1- Internal structure of the pump

2- the fact the matter is suction pressure (including the gravity affect) in comparison to discharge pressure is much much smaller so you need to have lots volume available for the suction.

3- Prevent cavitating the pump

Hope it helps.

To help prevent cavitation in the pump it is desirable to have a minimum pressure drop in the suction piping. One way in which this can be achieved is to have a larger diameter suction pipe. Of course, NPSH calculations can determine whether the larger suction pipe size is really necessary, but it is good practice to have it that way.

Hendrik is quite right, the type of reducer installed on the discharge piping can be concentric or eccentric, top flat or bottom flat. Please ensure though that the reducer on the suction side (if used) is always eccentric, top flat. This will prevent the formation of an air pocket in the pump suction piping.