Optimise media

When I optimise my media, the clip becomes really small in the playback viewer but it is normal size when I render out my timeline? Has anyone experienced this before & has a fix for this?

I've seen similar weirdness, but it never lasts. Just a frame or two looks this way.

You're not also using Proxy mode, are you? (Not the same thing as the new Proxy media option.)

Nope not using proxy mode, it’s really annoying I can’t seem to find anything that stops it from doing it