Audio files created using Voicemail Module and uuid_record API are having different size.

I made two calls, one to drop VM and other to record call using uuid_record API.

Size for VM file is 13.75291 MB/hour

Size of file created using uuid_record API is 6.866692 MB/hour

File format is mp3 for both files.

Both the calls were using G711a codec.

Can anyone help me understand why there is a difference ? 

Thanks in advance
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