The installed share presets are just "suggestions". You can always modify them according to your needs. 

If you would like to have much smaller files than the standard H264 export gives you, I would suggest to use the " Web Hosting" Format option. 

NOTE:
Please avoid "Computer" or "Apple Devices" as they don´t have a required timecode track. By choosing this format option it lets you export downscaled H264 files. This results in smaller file sizes. Unfortunately we're bound to the share preset architecture given by FCPX so tweaking the bitrate or using compressor presets is not possible.

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