How can I get file sizes for videos like on the iPhone

My daughter sent me a 12MB video from her phone of her dog jumping.

I edited out all but the jumps and slowed down the jumps by a factor of 4

The resulting video was 11 seconds shorter than the original.

But when I shared it as a 1080p Apple video the file was more than twice as large.

To make it smaller I had to share to FB.

Since I assume that file size is fairly closely related to duration when all the image parameters, scene complexity etc, are the same as in this case why is it that I can't get a file as small as the original one?

Do I need to buy Compressor?

Thanks for any help!

I would use the free programme Handbrake. It is very good at getting file sizes for social media down to a minimum

Handbrake is great but easier is to modify your export settings in the 'Export File' dialogue.
Set format to Web Hosting and codec to H264. See if that makes those files small enough for you.
If not Handbrake is a great no cost solution. Export from FCP as ProRes or ProResHQ to preserve best quality before converting it down with Handbrake. If you send something already heavily compressed to Handbrake the end result will be not very good.

What Carsten said

Carsten Orlt wrote:

Handbrake is great but easier is to modify your export settings in the 'Export File' dialogue.
Set format to Web Hosting and codec to H264. See if that makes those files small enough for you.
If not Handbrake is a great no cost solution. Export from FCP as ProRes or ProResHQ to preserve best quality before converting it down with Handbrake. If you send something already heavily compressed to Handbrake the end result will be not very good.

That actually made the file much larger, something like 93MB.

I don't see how to export in ProRes or ProResHQ. How do I do that?

I'll look into Handbrake thanks for the tip!

In the same dialogue where you selected 'Web Hosting' you can select the diff ProRes codec versions.