What bra size is bigger a or AAA?

The cup letter "A" is bigger than the cup letter "AA" when on the same band size.

The letter "A" means there is a one (1) inch difference between the width of the ribcage and the width of the bust, while the letter "AA" meands there is a zero (O)inch difference between those two.

This does not mean that all AA cups are smaller than all A cups. The cup size means nothing without a band size attached to it. They are dependent on each other. Someone who wears a 32A means their ribcage equals 32 inches and their bust equals 33 inches (a 1 inch difference), but someone who wears a 36AA means their ribcage equals 36 inches, and their bust equals 36 inches. So the person wearing a 36AA has a chest that is 3 inches larger than someone wearing a 32A.

When Bras were first invented, they were only in letters A, B, C, and D, and only on a couple of band sizes. When people realized that not all women fit into such a small amount of sizes, they added AA before A, and DD after D so those smaller than the A and bigger than D could wear bras as well. Several decades later, we now have about 20 commercially made band sizes, and 18 commercially made cup sizes, for a total of about 360 different bra sizes. Smaller bands (such as 24 and 26) and larger cups can be custom made either by hand or from specialty companies.