Here is a prompt-but-very-brief answer. Monarch 6 had an internal MDB that had a practical size limit of 1 GB total table size and 2 GB total MDB size. Monarch 7 provides support for "huge files," greater than or equal to 2 GB. The documentation states that there is "no practical limit" on the size of import input files.
If you need more info, I can point you toward the documentation.