It’s the same size as the 6 plus when it came out. It’s a huge difference.
The impact of the different aspect ratios is actually pretty huge. Since the Mini has a different aspect ratio than the iPhone 6 or 6+, for some tasks the screen is comparable to the iPhone 6+ and for some tasks the screen is comparable to the iPhone 6. For example, if you are watching a standard youtube video or TV show (with standard aspect ratio of 1.78), the video size is actually smaller than the iPhone 6 let alone the iPhone 6+.
Diagonal length is really not the best measurement to use when thinking about this, again because the devices here have different aspect ratios.