Automatic migration failed on Azure Database
The simple answer is don't use Entity Framework for this sort of thing. You need to write a SQL migration script, test it, and then deploy it onto your production system.  And yes - changing a table with 3M rows will take more time and resources than migrating a table with 3 rows.

There are a few techniques for achieving this. First, judging by the error message, you're trying to do this in a single transaction. Better to add a new column of the correct precision, copy values to it in batches of, say, 50,000 at a time, then drop the old column and rename the new one.

If the system has to remain operational during the migration you can get creative with views and triggers.