Image size reduced half way during animation

Rhino Rhino for Windows
1

I’m generating an animation by rendering the current view in arctic display mode for 720 frames.

However, half way into the rendering process (from frame 351), the size of the image reduced from 2382x1188 (1.427 KB) to 1804x828 (828 KB).

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So, in the final animation, the image size suddenly shrinks during half way.

Has anybody encountered similar issue? and May I ask what’s the solution?

Thanks.