It turns out that massive galaxies don't expand by making their own stars. Instead, they gain weight by chomping on nearby galaxies. Scientists have found that monster galaxies tend to absorb smaller ones in order to grow larger.
While small galaxies are very efficient at creating stars from gas, most massive ones are much less efficient at star formation. In fact, our own Milky Way is at a tipping point and is expected to now grow mainly by "eating" smaller galaxies, rather than by collecting gas.