Hey there! @Maryammoe, Thanks for stopping by HP forums!
I understand the video screen is bigger when you are watching youtube on your PC.
Don't worry I'd love to help you out.
Did you make any software or hardware changes on your PC?
Have you tried checking with a different browser? Does the issue still persist?
Please provide the product number and operating system you are running on your PC to assist you better.
Try holding Ctrl key + or - keys or use Ctrl + Mouse up or down to increase or reduce the size on your PC.
Also, try resetting the browser and check if it helps.
Please try the steps recommended below.
Google Chrome has an option that will reset itself to its default settings. To find this option, click the menu button (three horizontal lines) in the upper-right corner of the browser window and select “Settings”. Start typing “reset settings” in the search box. Settings matching the search term start displaying. Click the Perform a search for “reset browser” and you’ll see the Reset browser settings button.
The Reset settings dialog box displays, telling you what resetting your settings will do. Click “Reset” if you’re sure you want to reset your Chrome settings.