The 128GB limit shold apply only to hard drives on the internal IDE/ATA bus, and is applicable to original QS and earlier G4s, as well as G3s, etc.; it is not applicable to QS 2002 G4s and later. It is a limitation of the built-in IDE/ATA controller, not of the OS, and should not affect drives connected via firwire drive (nor USB drives, for that matter).
If that drive had ever been used and formatted on a machine subject to the limitation as an internal drive, it could well have been formatted as limited to 128GB. This should be able to be undone by reformatting (re-initializing) the drive.
Note, though, that OS 9's Drive Setup can not reformat a firewire drive. However, such can be done using Disk Utility on a machine running OS X. If that is done, be sure to select the option to install OS 9 drivers, and to choose Mac OS Extended (not journaled) as the format. Further note - this might be easiest of the OS X machine is running OS X 10.4 or earlier.
The question that Don raised was whether one of your Maxtor 300 GB drives had been formatted when installed on the internal IDE bus, in a Mac that doesn't support large drives (128 GBs->). If this was the case, it would still appear as a 128 GB drive when installed in the FireWire enclosure, in spite of the fact that FireWire-connected drives don't have that limitation. If you connected that FireWire housing and drive to a Mac that's running OS X or booted to the Mac OS X installer disk, you could use Disk Utility to reformat the drive to its full capacity.
Well I don't have a copy of X installed anyplace currently. So I took the "128 GB" disk and attached it to a spare port on my IDE disl controller.
I then initialized it with disk setup but did a custom initialization. That worked just fine.
I then plugged the disk into the Firewire case and now have a "300 GB" (269 GB actually) drive.