There is almost never a reason why I would want an image to be scaled past 100% by a transform. This usually results in an image being looking worse than it would if scaled by the browser. Is there a way to tell Craft not to scale an image past 100% of its original size, even if the transform width is larger?
You could also do the same test, and then conditionally setTransform on the image.
If you define your transforms in the CP the macro would be like this: