Ah, yes the size is in the layer's space because it can be oriented at any arbitrary angle, not just 90 degree increments. It wouldn't make sense if it was rotated at say 45 degrees - then which axis would you scale it on? Keeping the scaling relative to the layer allows for these arbitrary rotations.
You could always create a new blank raster-layer below the rotated image's layer and merge down, so that the rotated image is merged with the unrotated blank layer. The new raster-layer will then have the rotated layer natively oriented in the default canvas space, where the canvas XY axes are the same as the layer's.
- Charlie