Layer > Scale To Fit Frame reduces or enlarges the current bitmap layer so it will fit in the current frame.
The current aspect ratio and rotation of the layer is preserved, so if the frame's aspect ratio is different, the layer will only be resized until it fills the frame in one dimension.
To make a layer exactly fill the frame:
1. Do Layer > Reset Position.
2. Do Layer > Scale To Fit Frame.
3. Holding the Ctrl key down, drag the corners of the layer until they snap to the corners of the frame.