If you right-click those images and open them in another tab (or however your browser of choice works), the first image is half the size of the second image.
Far be it from me to think I can help an expert like you, but I'll try. Feel free to ignore me.
I'll bet you are on a 4K monitor. Me too, and I get the above problem a lot, but I've been able to avoid it because I'm careful with my image handling. Some apps scale images (including screenshots) and some don't. I once thought I could debug this issue by sending my images to Preview and examining the properties (eg, dpi) of the images there. But Preview is an app which scales images, so that didn't work.
Right now my only solution (actually, just a technique to help you understand where the fault lies) is to suggest that you experiment with changing the scaling of your monitor. MacOS has five levels of scaling available to us in the Display Pane of the Preferences panel. You might be able to make this problem go away (or at least change) if you try a different scaling (probably the fifth one prevents scaling.) I know that's not a solution... I only offer it as a way to help understand when/why it happens.
Ha! Yes, I know a few things, but there's way more that I don't know. I was originally a Windows developer, so my knowledge of Macs is hit-and-miss. And I never complain when someone tries to help.
I can definitely work around the issue. It actually causes a problem with KMFAM, but it just means over-large images in the Select dialog. No biggie.
But the reason I reported it is because I think it's a bug. I shouldn't get different results from those 2 menu items (at least I don't think so). And the issue with GIMP is just plain strange.
