Choruses are usually naturally a bit louder most of the time. People sing a bit louder and instruments are usually playing a bit thicker. This isn't always the case, but sometimes it is.
In mastering, this can push the compressor a bit more than the verses, which can actually lower the volume of the chorus by a bit. In this case, it might make sense to go boost the chorus by .5db or something like that... whatever the difference in compression levels happens to be.
