Last two columns made narrower in a Megillah
I recently got a tikkun for a 48 line meggilat Esther. The last two columns were made narrower, presumably to make sure the writing ended at the bottom of the column. Is this a requirement for a megillah? If so, what's the source? I have seen an older 50 line megillah where the writing ended in the middle of the column. Is it perhaps preferable to write columns that are all equal rather than have it end at the end?