James Leynse/Getty ImagesSmall-caps are the way to go in 2025, according to Heartland Advisors' Will Nasgovitz.Lower interest rates and deregulation are set to boost the asset class, the portfolio manager says.He shares 3 trades for investors to boost returns outside of the S&P 500.Bigger isn't always better. That's especially true regarding the stock market looking into 2025, according to Will Nasgovitz, CEO and portfolio manager at the $1.9 billion Milwaukee-based value-investing firm Heartland Advisors.Nasgovitz oversees various mid-cap and small-cap funds for the firm, and believes there are signs that 2025 is shaping up to be the year for small-caps and other unloved areas of the market, such as industrials and materials.Don't just take his word for it, though.