From the FAQ way down the bottom of the page:

"""

Will I get the same result as I would on a real device?

Sizzy simulates the viewport and user agent of each device, so the results should be the same as what you would see on a real device.

However, Sizzy cannot simulate different browser rendering engines, so there's a chance that there might be some minor differences when testing on a real device.

"""

"Exactly the same, except for the thing that makes them fundamentally different. Nothing we can do about that."

Even though it doesn’t simulate multiple engines, it simulates resolution, pixel ratio, user agent string, browser UI, OS UI, device frame etc. So it gives you a much more accurate representation of what your website would look like than any other browser out there.

Also, simulating devices is one of the features, even if you don’t care about responsive design, there’s plenty of features that can speed up your development work.

Let me know if you have other questions!

This should be a Chrome DevTools extension. On that note, isn't this feature already built into Chrome DevTools?

Sizzy started as an extension, but the extensions API is not powerful enough for all the stuff that we’re doing. Please check out what Sizzy can do before you say “this feature”, because there’s plenty of features that differentiate Sizzy from any other browser. We’re really trying to make the most useful browser for developers.

I've seen this in Firefox dev tools. Can't seem to find it now.

They moved it to the console under a gear icon.

Thanks. It's under dev tools > console > the phone logo > responsive

You can't view multiple devices simultaneously in Chrome