Sorry, I should've used a different word - you can't "expand" the image - i.e. make the size larger and keep the picture in a specific place, so that there's more space for annotation on a side. (or I'm missing the option?)

> Preview > Export > JPEG > compression slider

That's recompressing (losing the quality in the process) not optimizing compression. You can't update the compression settings on a png for example.