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GH-38745: [Integration] Fix huge integration test (#38746)

### Rationale for this change

For some reason, the decimal files generated by the integration test suite have as many record batches as columns.
This looks like a programming error rather than something deliberate.

### What changes are included in this PR?

Use similar batch sizes as for other test files.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

* Closes: #38745

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>