My ground sheet is plastic, so if I fold it this way, it will trap air and make it bigger. My solution is to fold only in one direction repeatedly until I have a long thin strip. Then I fold in a flap at one end, just a few inches. Then again and again, like rolling, only it's flat(ish). I squeeze the air out with each roll/fold, so it pushes out all the extra air as I go and ends up much smaller.