The pampas grass I have to cut back has grown in to a huge bush and expanded into a huge bush
Pruning Pampas Grass
This is a common problem with large ornamental grasses. The solution is to cut down the grass, dig up the entire plant and break off some new growth on the outside of the grass. A warning, this is difficult work and the rootball will be very heavy. I've used an axe to make divisions. Replant several sections of grass. They will merge into one plant. Then repeat the process every few years to keep the plant smaller and to make division easier.