Some models are susceptible to tank adjustments that will increase the flush volume.
Take the cover off and observe a standard flush. Then see if you can get more water out of the tank by holding the flapper control open.
If you can get more water out of the tank, then you can try to rig up some way to keep the flapper open. On my American Standard, there is a little marshmallow size float that I moved down to the flapper where it will keep the flapper open.
You can try it by holding down the handle to keep the flapper open until all of the water is out of the tank. If that works, then you can find an automatic solution.