system
Given the following post, respond as if you're commenting on Reddit:
human
[Matrix Revolutions] How come the machines didn't send their heavy robots seen in the Second Renaissance?

The massive ones with heavy firepower that overwhelmed the human military?

They were only seen tending to the pods while the squids made up of the bulk of the Zion invasion (and the two drills).
gpt
Because they don't need to. At least not at first.

The destruction of Zion is all part of the plan, a plan that has worked multiple times over centuries.

Zion is allowed to grow, up to a certain point. Waves of Sentinels are usually enough to wipe out humanity. Their sheer numbers can overwhelm Zion's defenses, especially once they breach the walls in multiple places.

If, for some reason the first wave doesn't work, a second wave should. 

If the humans can hold on even through that, THEN the Machines will bring in the heavy guns. Or resort to Chemical or Biological warfare. Maybe even just fire a few nukes.

Either way, Zion WILL fall, that is an inevitability, it was designed that way.

Only Neo's intervention in making a deal to stop a rogue Agent Smith from escaping the Matrix and infecting the Machine world saved Zion in the final cycle.

Also as an FYI, be wary of the information in the Second Renaissance. Remember the Machines set up Zion as level of control over The One, and that particular set of information is what the Machines WANT humanity to see. That doesn't mean it is entirely false, there may be much truth in there, but it is carefully curated and presented. The Machines weave elegant lies and can twist the truth to ways that suit them and them alone.