Funny that you mention resizing (Preview > Tools > Adjust size) and optimizing compression (Preview > Export > JPEG > compression slider), both tasks that Preview can do out of the box. For advanced edits I use Pixelmator Pro which has a UI that is actually usable compared to the decades-old cluster fuck that is the GIMP UI.
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.
