Ermotti returned to run UBS after the biggest bank merger since the global financial crisis was hastily arranged by Swiss authorities ten months ago to avert Credit Suisse's collapse.
Integrating the two arch rivals is a huge, multi-year task that will mean thousands of job losses in Switzerland and elsewhere and a single financial institution with a balance sheet around double the size of the Swiss economy.
Speaking at the World Economic Forum in Davos on Wednesday, Ermotti said while he understood concerns, some in Switzerland were being "indoctrinated almost daily by a lot of academics" to focus on the bank balance sheet size.
"If you look at risk-weighted assets as a percentage of GDP or as a percentage of our balance sheet, you will discover that the new UBS is de facto very low risk, very focused business model," he told CNBC in an interview.
Ermotti told a panel later on Wednesday that the acquisition of Credit Suisse, which has helped UBS shares soar more than 50% since, could be "the deal of the century" but only if the bank ensured integration went well.
"Are we larger? Yes. But are we stronger and more diversified? Yes...it's not a present, it's nothing for free," he said.
UBS Chairman Colm Kelleher said on Wednesday UBS' return on equity may be higher than its 15% target for end-2026, once the integration has been completed.
Kelleher also addressed concerns about the cultural fit of the two organisations.
"To a large extent a lot of bad actors had gone," Kelleher said in an interview with Bloomberg in Davos. "The people we have brought in, on the whole it has worked quite well; we have been quite surprised."
The moderator at Wednesday's Davos panel asked if Ermotti felt UBS had been "forced" to buy its rival.
"We were asked," Ermotti said, prompting laughter from the audience in the Swiss resort.
Switzerland's largest bank will announce a three-year strategic plan in February alongside its fourth quarter results.
Shares in UBS dropped 1.2% on Wednesday, with bank shares across Europe falling in line with weakness across stock markets.
