Here's why. You see, the so called "free syrian army" is no more than simply divided and broken up factions or groups of armed men. Al Qaeda isn't the proper word to use in Syria's case, ISIS is (Islamic State of Iraq and Syria) and they both bear the same mentality. Unfortunately, due to Iraq's devastated state, these group of jihadists have begun to form and grow strong particularly in Iraq. As the conflict began to rise in Syria, and president Bashar Al Assad refocused on whats actually going on 'inside' the country rather than 'outside', ISIS began its movements into Syria, and as they conquered more and more land, they would come across factions of the divided Free Syrian Army which they would either annihilate or force into recruitment. Some have began to automatically change sides onto the Jihadists due to fear of annihilation. The world's most dangerous mentality is spreading. Of course, upon the ending of the situation, a lot would change, since when you're forced into something, it wouldn't actually affect your mental beliefs.
Due to the inability of the upper echelons of Al Qaeda being able to control it's faction and elements within it, which has caused infighting like for example the Free Syrian army and isis, also isis has not long ago become a splinter group from al Qaeda and took there held areas ,personnel and resources with them, further sapping al-Qaeda's strength
I don't think al Queda is growing stronger in Syria. There are many reasons behind it. First of all the us military action in the Syria is growing stronger and I think thats the main reason. Also people in Syrica are getting stronger against al Queda and the terrism that they are facing.