It all depends on the manufacturer, it is not a low-flow thing. After the flush, water is supposed to be directed two places: fill the tank, and fill the bowl to overflowing. Obviously, they try to get the bowl level fill rate okay so they don't just waste more water down the drain. Most toilets have a hose from the fill valve directed into the tank's overflow pipe. This fills the bowl. If that hose is kinked, loose, or isn't running into the overflow, the bowl won't fill to the max. The bowl is like a cup with a hole part way up...you can only put so much water into it before it just runs out that hole. That is why HJ suggested to pour some water slowly into the bowl to see what that maximum level is - you may not be filling it. If the level doesn't change from pouring water into the toilet, then there's nothing you can do except buy a different toilet - that's the way it is. If it does go up and stays there, then look in the tank to figure out why it is not filling.