Not true.
Bitstamp publish their total volume only. MtGox don't publish a total volume, but splits it into individual currencies. If I deposit EUR on Bitstamp and withdraw EUR from Bitstamp, it is still counted as USD volume. On MtGox it would be counted as EUR volume. To compare the two you need to add all currencies on MtGox. Many people don't know this.
Well this seems to be mostly a discussion about USD and I believe the USD volume on bitstamp is now larger. In any case, the ratio of quoted USD volume has been tilting strongly away from Gox, likely due to a loss of faith in their business practices caused by their opacity, and probable falsehoods.