You are browsing Twitter.com and see an interesting image. So you just click on the image / media to view it larger. As you can see in the image, the size is 503 pixels by 510 pixels, above screen-shot was taken in Full Screen mode.
Twitter.com Viewing Image in Standard view
So you click on it to get a large view, but viola, twitter screws up all the real estate. When you click on the image, it opens up in a light-box in even smaller size 295 pixels by 413 pixels.
Twitter.com Viewing Image in Lightbox
By Design, user expects large view on clicking the media, here the opposite happens.
I saw this Twitter Blog post [Twitter Blog: Updates for viewing images] by @chenosauraus, targeted to solve exactly this problem. Guess it doesn’t.
There is even a Chrome plugin to solve this problem. Sad that Twitter couldn’t not solve it by design.
Please fix this @twitter.