The readme.txt says:
It is highly recommended that this game be run using
the interpreter that is supplied with it. KQ6AGI is very
resource demanding, and has been tailor-made to operate using
the supplied interpreter.
It would be interesting to know what's been modified in the interpreter.
There are no modifications. It's v2.917 (from KQ1). He just renamed it from agi.exe to kq6agi.exe.
It is big, but not the biggest. It's got 1,485,268 bytes of resources in two VOL files, uncompressed. Goldrush had 1,652,765 bytes of resources (compressed to 1,185,381 bytes as a V3 game).