A BBC article entitled "Battle for Iraq and Syria in Maps" gives an interesting overview of the waxing desire to be killing someone or decimating something else.
As with all such stories, the number of times the word "believe" is allowed a wide-ranging supportive role, as for example, "The US Central Intelligence Agency believes IS may have up to 31,000
fighters in Iraq and Syria - three times as many as previously thought." (Italics added) Stay tuned: It's a good bet that 31,000 will become a stated fact in coming weeks.
No where have I found anyone stating how large IS or al Qaida actually is. I guess that would risk being laughed out of town -- to think such a meager force could actually damage or defeat the US or its combative allies.
Back before al Qaida loomed large on the enemies list, there was a film clip showing the newly-discovered Osama bin Laden carrying an AK-47. He was pictured with a pickup clutch of men said to be al Qaida fighters when in fact that AK-47 was borrowed and the men had been gathered up locally for a photo shoot. Osama was a creature of western convenience, much as it is hard not to suspect that IS and its (oh dearie dear!) "31,000" fighters aren't equally convenient: After all, they have yet to pose a physical danger to the US that is bombing the shit out of them.