The essence of an animation is that it is a sequence of matching frames with gradually shifting contents. It makes sense only when the frames match--that is, they are the same size. Therefore the complexity added by the frame_size_can_vary attribute is not buying anything, and should be removed. Specifically, what it allows is writing a sequence of png files, for example, which cannot subsequently be assembled into a video. Even if viewed sequentially with a png-viewer, such a sequence will not be satisfying because the screen position of features that should be stationary will jump around with the changing frame size.

It was put in to support bbox_inches='tight'. I suggest that this kwarg setting should instead raise a ValueError. By requiring that the frame size be fixed from the beginning, it becomes easier to handle problems such as the need for even-number pixel dimensions with h264.