First of all, Unit Entitlements (UEs) are not calculated solely on the size of an apartment and, in fact, bear closer but not exact relationship to the value of the property. Thus a three bedroom unit at the back of a building can pay less in levies than a single-bedroom unit on a higher floor with views.
However, if there is a serious discrepancey, for instance where UEs have been deliberately reduced to favour one vendor – usually on a commercial space or a penthouse – you can petition to have them changed by a tribunal but it is a hideously expensive process involving surveyors, accountants and lawyers.