There are a number of things you can do to fit more waste in your cardboard box. Collectors often find that placing weights or heavy items on top of your waste overnight, can help to compress it so you can fit more in the box.
You can also use an old plastic bag or bin bag to compact flexible packaging like packets or wrappers.
Other collectors have also suggested placing your waste in plastic bags and vacuuming the air out of the bag. This creates more space and works well with flexible packaging such as crisp packets or biscuit wrappers.