@starburst98
They could carry a small amount of elite monsters on their back to cover their blindspots while also being supported by the flying troops.
Then they would just smash through the walls and devastate whatever is in their way.
After they have run some way beyond the fortress they turn around.
Now they can attack the fort from behind while other monsters storm from the front and the flying monsters attack from the sky - a three-pronged attack.
Instead the giant monsters carry lots of small monsters which make them unable to really run as they have to watch out for those large groups on their backs.
Then they only hit the wall until a part grumbles and need to stop then to let the smaller monsters down from via their heads.
So they are then actually forced to not move their heads in an exposed position.
A head is actually a weakspot I'd say. So having to hold still to let them down is doing exactly the opposite of what you say the small monsters are meant for.
They do not protect the weakspot called "head" they are the cause that it is especially exposed to counter attacks in that situation.
To compare it with your tank example.
It is like tanks carrying foot soldiers to look out for some enemy foot soldiers who might try to approach from blindspots.
But the tanks are attempting to attack an enemy artillery position and just before driving up the hill to those artilleries they stop to let down the foot soldiers so that the foot soldiers can storm it instead.
That does not make sense.
And that is exactly what the monsters are doing here.
Edit:
@Robbini: Check this somment above this line. That is what they should go for.
The way the monsters are attacking makes the combo of small + large monsters useless as they combine their weaknesses together instead of their strengths this way.