Bohr Model and Exciting Electrons
The energy difference between the orbits gets smaller the higher "up" you go. 

For the classical bohr model the energy of a given orbit $n$ can be approximated by:

$E_n=-\frac{Z^2R_E}{n^2}$

where $Z$ is the charge of the nucleus and $R_E$ is the Rydberg energy. Since there is a $n^2$ in the divider the difference $E_{n+1}-E_n$ gets smaller and smaller.

This is explained in more detail here.

As for why this is the case: here we have to look into quantum mechanics and I guess this is outside the scope here. But if you are interested look up explanations of a particle in a 1D box. The spacing of the energy levels there follows the same trend and you can find explanations there on why this is the case.