The reason the method seems to catch smaller shoal fish is because it was designed to cause a competition for particles buried in a ball of groundbait.
When used properly with regular casting it can cause a frenzy with fish competing for the bait as it hits the surface and following the feeder down picking off lumps and particles as they break away from the main ball.
In this situation the larger fish will often be on the outside of the shoal. This is not, as many people believe, because they are more wary or cleverer, it is simply because they are not as agile as the smaller fish and struggle to compete in this situation, think of a big ship surrounded by speedboats.
The way to pick off the larger fish is to dump the short hook length which is usually associated with the method and use one around 2 foot long with a high visibility slow sinking bait.
Don't tuck the bait into the ball as you usually would when fishing the method as the idea is for it to fall away from the main action and into the range of the larger fish on the outside of the action.
Give it a go and let us know how you get on.