Hi @Mike_Z you are probably timing out so perhaps try.
wait_for_completion=false
Which is really what you want to do anyways. You will get a task id which then you can check the status using the task API
Although 150mb is pretty small so perhaps some other reason but that is what I would try first.
You are not guaranteed the resulting index will be the exact same size in bytes as the source as they may have a different number of segments etc.
Unless you force merge both down to 1 segment after everything is done will they be the same size.
I would look closely at the documents as there are many options to optimize reindex for large indices