I’m guessing with loose math here. But I think 3GB for 25 minutes is about 2,000kbps bitrate.
Which is really low. That’s like what you’d use for streaming 30fps video, on good hardware, with some quality settings cranked up to counter the low bitrate.
If you can deal with the render times, I would export your video from final cut with a common lossless format. And then compress it with something which exposes more options for you.
Handbrake works well and allows you to use your GPU for faster compression.
However, this thread is largely about how we have to trick youtube to give us better quality. youtube re-compresses everything you upload. And it does it based on resolution and framerate. Higher resolutions and higher framerates, get more bitrate. And anything lower than 2K resolution, gets butt for bitrate.
Basically, I suggest upscaling your video to at at least 2K/60. If not 4K/60 (even if its a 30fps source. frame double it to 60). Before uploading it to youtube. and using a fairly high bitrate. Because we want youtube to compress from as high a quality original, as possible. Some people with amazing internet connections just upload lossless files…
I’ve been using a program called Video Enhancer 2, which does very good quality upscaling. Its a little bit finicky, but I’ve got it figured out. Its totally free. even after the trial period, you just give your email address and they hand you a free code.
However, it runs on your CPU, so it will take a long time. I’ve been using that @ 80,000kbps and the quality is really great.