While this won't help with batch Paste Size operations, for single images, a workaround is to manually reduce the size of the destination image to make it smaller than what you intend to by paste sizing. Then, when you Paste Size, the image will be rescaled up when you Paste Size.

Not an ideal solution, but until this is fixed, it's one way to do it. I am actually hoping this is a bug and not a "feature". It certainly does not match the description as outlined in the help file.

Note: I have logged a technical support ticket with Magix/Xara regarding this issue to see what they say.