Why do quantum objects slow down when volume increases? I was reading this article about the EMC effect and saw this quote

“In quantum mechanics, anytime you increase the volume over which an object is confined, it slows down,” Schmidt says. “If you tighten up the space, it speeds up. That’s a known fact.”

And was trying to figure out why and haven't been able to find a conclusive article or answer. MY guess is that the interactions with other quantum objects is where they get most of their velocity, thus greater volumes means less intense interactions?
 A: It's just like musical instruments: the bigger the instrument, the lower the frequency and the longer the wavelength of the sound that leaks out of it. In the quantum world, momentum is inversely proportional to wavelength. So, the lowest modes will have lower momentum/energy in a bigger box.
A: This is a loosely-phrased consequence of the uncertainty principle, which says that the product of the position uncertainty and the momentum uncertainty is larger than some minimum value that you can find from Fourier analysis of the wavefunction:
$$
\sigma_x\sigma_p>\hbar/2
$$
One way to compute the uncertainty in an observable is to take the difference between the mean of the square and the square of the mean:
$$
\sigma_p^2 = \left<p^2\right> - \left<p\right>^2
$$
If I have a particle in a "small box," then its position uncertainty is small. If the box is stationary, then the mean momentum is zero, $\left<p\right>=0$. The uncertainty principle requires that the mean momentum-squared, which is related to the kinetic energy $p^2/2m$, must be large.
If you solve the Schrodinger equation for a particle in a box, the size of the box shows up in the denominator for the energy of the ground state — a result directly related to the uncertainty principle.
Note the converse, as stated in your question, is allowed rather than inevitable. If you have a trapped particle and you make its trap bigger, the particle may transition to a lower-energy state which didn't exist in the smaller trap.
