Politicians do not seem to be good at imagining unintended consequences. The cap on bankers’ bonuses, however popular, may be counter-productive in reducing risk.
If a smaller proportion of a trader’s income is at risk if a trade goes wrong they may pursue high-risk opportunities to get that big win. If it works they guarantee the full bonus and could also use it to negotiate a higher salary (and bonus) for the following year. If it fails the downside is limited by the capped bonus to a smaller part of overall income. At the end of the day traders are competitive and gamblers at heart so will they be more likely to pursue the big win when their own risk is limited?