For the same reason why the 50-cent coin is greater than the 1-euro coin, or earlier the 10-penny coin larger than the 50-penny coin and the 2 Pfennig coin larger than the 5-penny coin was: so that you can distinguish them better!
Based on color and size you can easily see without exactly what coin it is. If the different coins would grow up with increasing value, either the small coins would be too tiny, the big coins too huge, or the size differences too small.
The differences go to color ( Type of alloy) and by size. Within a color, there must be size differences to make the coins easier to recognize. That's just 5 cents larger ALS 2 and 1 cents. Comparing with the 10-, 20- and 50 cent coins does not matter because of the different color to distinguish.
The diameter is so large to choose You can get it easier to get out of the purse.
To make the better of 1 and 2-cent Much can distinguish coins