ffmpeg – Why is my AV1-encoded video (libsvtav1) larger than the original H.264?

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I’m trying to compress a batch of 10 H.264 videos (each ~5 minutes long) using the SVT-AV1 encoder via FFmpeg. Surprisingly, for all of them, the resulting AV1 file sizes are larger than the originals even when using -crf 24 or -crf 30, which I expected to reduce file size.

Original Video Info (H.264)

File: 16.mp4,
Codec: H.264 (High profile),
Resolution: 1280×960,
Duration: 5:00 minutes,
Bitrate: ~1463 kb/s

FFprobe output: Stream #0:0: Video: h264 (High), yuv420p(progressive), 1280×960, 1460 kb/s, 30 fps

Encoded AV1 Video Info (Example: CRF 24)

Codec: AV1 (libsvtav1),Bitrate: ~1503 kb/s

ffprobe output: Stream #0:0: Video: av1 (Main), yuv420p, 1280×960, 1501 kb/s, 30 fps
encoder: Lavc62.0.101 libsvtav1

Encoding Commands Tried (on all 10 videos)

ffmpeg -i input.mp4 -c:v libsvtav1 -crf 24 -y output_crf24.mp4

ffmpeg -i input.mp4 -c:v libsvtav1 -crf 30 -y output_crf30.mp4

What could be the possible reason for such behavior?

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