I would like to have a look at your scene file. I can send an upload link.
(Dropbox, Amazon, Apple, Adobe, Google, Wetransfer is fine, as long as the URL is not shortened)
What you could do, first, check if the Filter in the picture viewer is on. If so, that might be the problem.
Perhaps do a little test:
• Render in the Editor view and then go to:
• Editor View Menu> View> Send to Picture Viewer.
• This gives you the size of the current editor view rendering.
• Take these numbers into the Render Settings and render an image to the Picture Viewer.
• Now make an A/B comparison between the two.
• Eventually, go the Queue route and add one more image to the list in the Picture viewer.
Do another A/B
If that all fails, I can have a look at your file, or you go directly to the tech support:
The two renderings will typically come close, but not as far apart as in your images.
All the best