The amount you pay in the sinking fund is agreed by the owners each year at your AGM (often on the advice of your strata manager). You have a vote and you have a say so this is the time to raise any concerns.
The amount you contribute is related to your Unit Entitlements (which also relates to your voting power should a “poll” rather than a straight vote be called for at a general meeting). Your Unit Entitlements are based on size and position of your apartment and are therefore very loosely in line with its value.
The law in NSW relates all payments to the admin fund and sinking fund to the Unit Entitlements. In other states they split the contributions so that some are calculated per household and others by unit entitlements.