What does “stronger” mean, and what does “considerably” mean?

Qualification makes the argument stronger, but it does not mean that your original argument was weak.

Here, stronger means more defensible against reasonable historical objections.

For example, suppose you say:

“Capitalism turned workers into the middle class.”

Someone could object:

“Not all workers became middle class.”

But if you say:

“Some workers were able to improve their economic position and enter the middle class.”

that objection no longer defeats your argument, because you have already specified some, rather than all.

So:

stronger argument is an argument that makes a claim of appropriate scope and is therefore harder to refute.

considerably stronger is substantially more defensible, not merely slightly better.