I suspect this is a limitation of 32-bit addressing for either Cognos or Windows - it might be an answer that Cognos support can give without too much effort on their behalf.... :^}
Notwithstanding the issue of manipulating a 2GB+ cube, have you considered some degree of data consolidation prior to build or perhaps enabling drill-through from a master cube to some ancillary ones?
soi la, soi carré