ffmpeg reducing 60fps to 30fps, but file size is larger
Did you actually get every second frame? My impression is that you got every frame, but you halved the framerate. Therefore, the length of the video in seconds is twice as long for the first compared to the second, and assuming equal rate controls (e.g. 1 megabit per second), the twice-as-long-file will thus be approximately twice as big, which is what you got.