How to Change # of Pixels Width at Full Screen

I have a weird problem, I'm not sure my title summarizes it well:

I was using Figma to design my website. Then, when I begin to implement the design, everything was too big. No big deal.

Then I realize that in Figma I was working with a 1440x900 screen. So I check chrome dev tools to see the width of my screen is when it's full screen: 1152px. I check some other websites to see what their width is at full screen. Instagram: 1440px, Reddit: 1440px, Google search results: 1600px.

Why is there this difference? I am not changing the actual width of my browser window, yet chrome dev tools is saying these sites have different widths measured in pixels. How can I change my maximum width in pixels?

I am using React, Nextjs and Chakra-ui, if that makes any difference.