Footage loses quality when I insert into timeline?

Hello guys,

Hope you're all well. My first post on here

Not the best with Premiere and I've encountered a bit of a problem, and hopefully you guys can help as I need it finished asap.

I've imported some GoPro footage into my project, and on the left hand preview box it is much better quality than on the right when I insert into my timeline.

When I insert the footage on the timeline, I have to decrease the size of my footage as it's majorly zoomed in, so maybe something to do with the size is the problem, but I'm no expert.

I've inserted a photo, but it's hard to really notice the quality difference, but I can assure you there is.

Hopefully someone can point me in the right direction.

Thanks guys!