Video Blury When I put a large file in windows movie maker?

I made a minecraft video with the resolution 1080x720p. The file is an MP4 file with a size of 228MB. The video is 4 minutes long and when I put it in movie maker, it says that it's preparing files in the bottom left corner. After they were done preparing, I checked the video quality and my 720p video was a 480 video. I don't know what to do because before this video I edited another video which was 2 minutes long and it's size was 100MB and it didn't lose quality.

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