it makes sense if you think about it- you can actually do damage to your breasts glands if you use the wrong size flanges over a long period of time. the right size is key. only your aerola should be sucked into the tunnel of the flange. no fleshy skin. by just having your nipple suctioned- the milk will be drawn from the breast more accurately- and do less damage to the mammary glands.
You may be getting too much breast tissue drawn in with the larger flange like PP said. Only your nipple and just a bit of the aerola should go into the shield. If too much breast tissue is going in, it will compress the milk glands too much resulting in less milk. If you use Medela products you should really call and talk to a rep about the shields they are SO helpful and will send you new shields in the correct size.
Re: larger shields, less milk?? WTF?