Why is my 26 minute Final Cut Pro slideshow 62 GB in size?

FCP likely isn't the best format for this..

The FCP document is likely to be bloated since it contains all the media and metadata around your slideshow.

Typically you'd export the FCP document to a movie, which would be more compact. A 25 minute video should be a few GB in size, at most.

Alternatively, use something like Keynote, which is optimized for slideshows, and tends to be more compact than FCP

FCP is perfectly fine for this. Most likely the problem is you have background rendering turned on, which is generating large high resolution ProRes HQ media. In FCP settings switch off background rendering and leave it off. Select the library and use File > Delete Generated Library Files. Delete everything listed. How much space do you recover?

I once created a 6 minute memorial slide show and music using FCP. The final product was a 1080p at 23.98 frames per sec, encoded H.264 at 19 megabit/sec. I think I used the FCP preset File>Share>Export File>Settings, Format: Computer, Video Codec: H.264 Single-pass, Resolution: 1920 x 1080. The file was 846 megabytes. Scaling that to 26 min (26/6 = 4.3), the equivalent file size would be 3.67 gigabytes.

Do exactly as Tom says and here is a little extra info.

FCP by default renders everything you do automatically.

These render files are gigantic and can quickly fill up your drives.

Rendering is done to make your computer handle the files easily especially if it is low powered.

However, pretty much every Mac made in the last decade is powerful enough to not require those render files unless you are doing something particularly intensive.

So to avoid creating those massive ProRes render files you can turn off "Background Render" if you go to Final Cut Pro/Settings/Playback and deselect the "Background Render" tick box.