Pulsar's answer is indeed correct, but let me expand a bit more.
What happens when a gas giant shrinks?
A uniform mass will have a self gravitational potential of $-\frac{3GM^2}{5R}$. If we decrease its radius, its potential will decrease as well; and the difference will be turned into thermal energy. Although, gas giants and stars are not uniform mass balls, their gravitational binding energy is still proportional to $\propto \frac{GM^2}{R}$; so if the radius decreases it will release energy, which will rise the temperature in return.
What happens when the temperature increases?
Assuming the gas in those planets obey the ideal gas law $$PV=nRT$$($R$ is not the radius here, it is the gas constant ($8.314\,\text{J·K}^{−1}\text{mol}^{-1}$)), it's obvious that when $T$ increases and $V$ decreases(due to the shrink in the previous section); $P$ must increase. Note that most real gases behave qualitatively like an ideal gas, so this is not a wild crazy assumption.
So what is the big picture?
The planet shrinks a little bit, the potential difference turns into thermal energy and its temperature rises. The rise in temperature will cause the pressure to rise and prevent the planet from shrinking further(hold the planet in hydrostatic equilibrium). But the planet looses energy due to EM radiation as well, so it will continuously shrink and radiate. The process is called Kelvin–Helmholtz mechanism.
For instance, Jupiter is shrinking the tiny bit of $2\,\text{cm}$ each year. Although you might think this is really nothing, the amount of heat produced is similar to the total solar radiation it receives.
