Rotational Energy Levels Why does the rotational constant B decrease and transition spacings decrease as the mass of a particle increase? 

I understand from a purely equation perspective that since

$$B = \frac{h} {8\pi ^2 cI}$$ 

that as $I$ increases the denominator increases and so $B$ decreases. But what is the physical reasoning behind this? Why or in what way is the rotational constant dependent on mass?
First, take a look at classical physics. The angular momentum of a particle rotating in a plane is defined as
$$L = I \omega$$
and its kinetic energy is
$$E = \frac{1}{2} I \omega^2 = \frac{L^2}{2I}.$$

So if you formulate your energy in terms of the angular momentum of your rotating particle, you arrive at the inverse relation. 

In analogy to the classical picture, the eigenvalues of the rotational Schrödinger equation,
$$ E = hcBJ(J+1),$$
likewise depend quadratically on the angular momentum quantum number, and thus have a similar inverse dependence on the moment of inertia through $B$.