Since any piece moved to the opponent's side becomes the opponent's piece, it is not possible to deliberately reduce your opponent's army—only your own!
... A capture decreases the size of your own army by one while leaving your opponent's army the same size
But none of that is necessarily a bad thing. Didn't anyone ever tell you that size isn't everything?
The purpose of the game is to capture pieces. Your opponent having more pieces is not necessarily a better thing for them. If you manage to capture a higher value piece with a lower value piece, you've both scored points for yourself and weakened their army.
If you can get far enough ahead, you can afford to take less optimal moves (including just moving pieces over to the opponent's side of the board) in order to end the game.