Generally these “shrink the economy” arguments are based on making assumptions of what GDP would be at some point in the future, making a guess about how much illegal immigrants contribute, and then subtracting.

But hypothetical growth that doesn’t happen is not a “cost” to the economy. Nor is it necessarily the case than a smaller than projected GDP along with a smaller population is a net negative.

If a scarcity of workers on the low end of the pay scale leads to higher wages, then the effect on low income workers would be positive.