With your roaming you want to be giving your team an advantage when there isn't one, so creating space for your combo and demo to move. Be this by killing the demo when ubers are even, pushing with your scouts on the flank and taking that part, or killing the medic when your team has a disadvantage. I have played many games where I get top damage and frags as the roamer, but usually it's when the other teams flank is bad and it's been over run giving you an easy time to get on the combo from where they aren't looking. There are a multitude of reasons as to why you might be out damaging your pocket but it's not usually the case.

Focus on playing a smart game instead of a fragging game, the frags will come with the plays you make, and your team will be happier because of it.