Gas giants start big and luminous and get smaller and less luminous.
Their gravitational contraction gives them a luminosity. The virial theorem then demands that they must contract in response if their gas pressure is a function of temperature.
However after evolving for some time, the cores of gas giants become partially and then nearly fully electron degenerate. A degenerate gas can cool whilst maintaining its pressure. As a result the contraction slows and tends towards a cold equilibrium size. Throughout, this evolution the luminosity is approximately proportional to the reciprocal of the age.
The following picture was taken from the review of Fortney et al. (2011, in Exoplanets, edited by S. Seager, Univ. of Arizona Press, p.397), showing the radius evolution for various compositions.
There are many, many details beyond this very rough sketch. The whole field, as you might imagine, is extremely active at the moment, thanks to the discovery of many giant exoplanets. Complications include how do they form; what are the compositions; how do chemical elements separate; how is tidal heating and irradiation felt; what is the high pressure equation of state; what are the cores made of?
Useful recent reviews include
Guillot & Gautier 2014
Baraffe et al. (2014)
Fortney et al. (2011)
Fortney & Nettelmann (2010)