When using the background masking of Mtext, the extents of the box often reflects the window used when creating the mtext. To resize this box to the minimum, double click the mtext object to bring up the mtext editor, then double click the <> icon in the ribbon that is just above it in the ruler bar.
If you have multiple mtext object to process, ToolPac contains a Mtext Minimum Size tool that processes a selection set.