That is not entirely true - Brussels Moslim population is 256,000 - Mecca on the other hand is over a million.
Wrong (1) it's 512,000. Half the Muslims, being women, are in strict purdah, lest an infidel gaze upon their alleged charms. (Muslims are like mice. For everyone you spot, etc. etc.)
Wrong (2) Mecca only hits a million during Spring Break