Final cut pro export file size is huge (despite the small size of the initial video) - how can I shrink it?
A Master file export is a multipass high-resolution export. Its meant for archiving or transcoding into other formats.
Also, your Zoom recording is probably in a format what is not natively compatible with FCPX and was transcoded into ProRes during import. ProRes is excellent for editing, saving space is not its purpose.
Depending on where you want to publish your video you should choose different export presets, what will trade size for quality or vice versa. However, I doubt you can export in the same format than the recording from Zoom, so export size might vary significant.