Most dual flush toilets are dual flush because there is very little water in the bowl. It's sort of a European thing.
There is no adjustment for it, it's dertermined by the trapway bend.
You will notice that no matter how much water you pour into the bowl, it will remain at the same level.
The bowls tend to get a bit better in a month or so.
The first few days are kind of sticky.
I use a liquid bowl cleaner without abrasives.