file.acc is 50% bigger than source file.mp3. Is it ok?
Default option settings will be used if you do not choose a quality or bitrate. For the
libvo_aacenc encoders this will result in ffmpeg automatically using
-b:a 128k. Since these encoders only accept a bitrate instead of a quality level then you must adjust the bitrate value higher or lower to get your desired output quality or file size. Alternatively, depending on your ffmpeg build, you can use another AAC encoder, such as
libfaac, that does allow you to target a specific quality with the
Or are there any options to convert to smaller size without loosing quality?
Technically, no. This is because you are going from a lossy format to another lossy format. General recommendation is to avoid re-encoding. Depending on what you want, and if you must re-encode, then the goal is usually to use a good enough quality that you hopefully do not notice a change. If file size is important then you must find a good balance between quality and file size. Quality is subjective so I can not tell you exactly what will work for you, but you can find some useful information and examples for each AAC encoder supported by ffmpeg at the FFmpeg and AAC Encoding Guide.