Without better information on the whereabouts of the explosion, it's hard to draw any conclusions. But from the photos I've seen, any crowds targeted by the bomber(s) would not have been due to security lines, but rather the densely packed lines at the various airline checkin desks. Unlike most American airports, where broad desks face the outer wall, at Zaventem they're arranged in facing rows like booths at a convention.
Any security lines are actually past passport control, and I've never seen them particularly packed.
Moreover, the bomb was set off outside the secure area. It is VERY likely that existing security measures got more people killed because there are dense crowds waiting to clear security. This exact type of attack, outside the secure area, was predicted long ago. And there you have it.