Any move by the Australian Prudential Regulation Authority to require the big banks to increase their risk-weightings for determining capital adequacy would weigh most heavily on Westpac, according to analysis by Goldman Sachs.Earlier this month, the Basel Committee on Banking Supervision issued a report on the regulatory consistency of risk-weighted assets, saying there was considerable variation across the industry globally. It wants to see consistent application of the Basel III rules.Goldman Sachs has "stress tested" the big Australian banks to see what would happen if they had to increase their risk-weightings. Its report said Australian banks had strong capital adequacy levels, but the Basel Committee study has raised the prospect that there could be upward pressure on risk weightings.The report said: "We think the risk that the Australian Prudential Regulation Authority will push for higher risk weights is low and we also note that, across the major risk classes, Australian banks' risk weightings are at or above global averages."The risk weightings of Australian banks to the major credit classes remain at or above global averages. This is despite the fact that Australian banks' retail portfolios are significantly more overweight [in] lower-risk mortgage lending that their global peers."However, given the high concentration of mortgages on [their] balance sheets and their adoption of the advanced IRB [internal ratings] methodology for calculating risk weights, the Australian banks could be vulnerable to any global measures that sought to more closely align advanced IRB capital requirements to standardised calculations.""Our analysis suggests that every five per cent increase in risk-weighted assets, as a result of higher risk weights, translates into a reduction in sector returns of about 0.7 per cent."Given its relatively larger and more optimized mortgage book, we see Westpac as the most exposed to any shift in risk weightings. This supports our Sell recommendation."Over the past 12 months, a number of regulators have pushed risk weights higher. These include the Reserve Bank of New Zealand, as well as the Swedish, Norwegian, Danish and Hong Kong regulators.Goldman Sachs tested four scenarios. The most moderate assumed that a floor was set under risk-weightings equal to the global median, as documented by the Basel Committee. Australian major bank capital ratios would increase by about 20 basis points.Under this scenario, Westpac's risk-weighted assets would increase by five per cent, compared with one per cent for ANZ, three per cent for Commonwealth Bank, and three per cent for National Australia Bank.Westpac's return on equity would fall by 0.8 per cent, compared with 0.1 per cent for ANZ, 0.4 per cent for Commonwealth Bank and 0.4 per cent for NAB.In the most extreme scenario, the banks' calculation of risk-weighted assets reverts to a standardised approach. In this case the capital ratio impact would be more than two per cent; risk-weighted assets would increase by 40 per cent; and return on equity would fall by an average of 4.5 per cent. Again, Westpac would feel the impact more than the other big banks.Goldman Sachs rates NAB a Buy, ANZ and CBA