Placing the atomic mass and number of an element...
In the periodic table, the largest number is indeed the atomic number. This is the number of protons. Since this defines the element, it is considered the most important number.

What you probably mean is 
$^A_Z\mathrm{Element}$. Here, by convention the lower $Z$ (from the German word Zahl is the atomic number, while $A$ is the sum of protons and neutrons. This defines a particular isotope. This notation is often used in the context of NMR spectroscopy ($^{13}\ce{C}$ NMR), or when radioactive decays are described:

\[\ce{^{14}_6C -> ^{14}_7N} + e^- + \bar\nu_e\]

In summary, I think this is just a set of different conventions which show that in differents contexts either the element itself or a particular isotope and its properties are considered more important.