Jibberish.
If it looks good, it IS good.
Top and sides are the same size and IF you want, the bottom can be 1/2" or more larger... ORRR.. you can make them all the same size. The biggest thing that makes an image look bad is to make the mat width too small in relation to the width of the frame. The basic rule (starting point) is a 2 to 1 ratio. If the frame is 2" wide the mat would be around 4" wide +/-. All that is a general rule and is meant as a starting point. You are free to adjust sizes till it feels right.
A "golden mean" rule of matting is nonsense and I dare you to find more than a handfull of framers across the country that use this for matting work.
The one thing that really looks bad is to have the sides the same size, and then the top another size and the bottom yet another size. It screams out amateur who hasn't a clue.
Rule number #2 is to ignore Rule #1 whenever necessary.
Saying all that... if YOU like it, then it is right FOR YOU.
...and yes, I'm grumpy, so what?