Tried scaling a diagram with a gradient background and it seems that the background rectangle is scaled twice. For example, this simple diagram at half scale ends up with a gradient which only covers the top left quarter of this diagram:
skinparam backgroundcolor blue/red
Bob -> Alice : hello
Other directions of gradients all end in the same corner. The same diagram at 10x scale clearly scales the background rectangle way off screen such that there isn't a trace of red in the diagram.