All clips are more zoomed in now???

Hello,

I'm a relative noob to all of this. So I do apologise if there is a basic solution to this. 

So I spent quite a bit of time making a music video for one of my songs, and I'm nearing the end now. However, after looking back through my video, I've just discovered that everything is much more 'zoomed in' now. As in, the scale seems to have been adjusted somehow?

I'm not sure why. Here's some possible relevant information;

I have just added some 4k clip to what was previously just 1080p footage. Could that have affected things?

Also, when adding these clips, I selected the 'scale to frame' and 'set to frame' options in an attempt to centre them.

I wonder if this has something to do with it?

Anyway, I spent ages adding key frames to adjust the scale of certain clips etc, and I can't be having to do all that again.

Could someone please tell me what on Earth has happened please? And how to rectify it?

Many thanks.