When printing 2(4,8,etc)-up, everything Just Fits, because of the 1:sqrt(2) edge length ratio. Similarly, A3 is suitable for making A4 size booklets, and many big photocopiers can do this automatically.

Also, because an A0 sheet is defined as having an area of 1 square metre, and paper weight is in grams per square meter, you can easily calculate the weight of any number of any A-size sheet.