Similar to an even exchange in chess. If you’re behind it’s a good deal, if you’re ahead it’s bad. Of course evaluating value taking into account positions (and thus determining whether an exchange is really “even”) is not straightforward.
Am I missing some reason why it would be the opposite?
Let's say your "strength" is simply the sum of the value of your pieces. The losing side has a strength of l and the winning side has a strength of L.
The losing side loses by l-L in absolute terms, or (l-L)/(l+L) in relative terms.
An even exchange of value k makes it go to (l-k)-(L-k) = l-k in absolute terms (no change); and to (l-L)/(l+L-2k) in relative terms. (l-L)/(l+L-2k) < (l-L)/(l+L).
To take an example, if the situation is white : two pawns and black: one pawn, going to white: one pawn and black: nothing is a bad deal for black (the losing side)!