usability - Fullscreen modal vs. conventional modal?

My site comes across many instances where I'd like to use a modal. Sign up, sign in, comments, and file uploading are a few examples.

Now, I can go with a conventional modal design where the modal is typically fairly small in size and you're able to see the page in the background (behind a transparent black overlay), like the one below:

Or I can go with a fullscreen modal design, like the one below. I find that fullscreen modals look better and cleaner.

What are the disadvantages of using a fullscreen modal? Should I go with the conventional modal design or the fullscreen modal design?