So it's the same display in both cases? He's connecting the TV to the laptop?
If you want my guess the laptop is NOT 1080p. Since theere are fewer pixels being output by the laptop, the stream takes up more of them.
Let's say the streaming HD content is 720p. And let's say his laptop has a 900p height. When the desktop is displayed on the TV, the 720p takes up 7/9ths of the visible area.
Now the new GPU is set to a higher resolution, so the default view of the NFL stream takes up LESS of the total area.
Does that make sense, and does it answer the question?
If the laptop is really providing the same 1080p.... then it's just a matter of the default view in Windows or the browser.