I find it unlikely that the UBB is directly responsible for that jump in disk space use. A fully cached UBB will take about 25-40% more disk space than a UBB5 (as there's more HTML and more stuff being written to disk)... but a full cache on a board that is frequently posted to will never, ever occur.
Let's do a little investigation. Is this a Unix server? If so, do you have shell access? And if so, do you feel comfortable using the shell?
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Charles Capps
Programmer, Infopop Corporation
Please do not contact me privately for support - post on the board or open a support ticket instead!