In general, large companies have a lot more data around them, a lot more coverage from analysts and journalists and the like, and generally have a lot more eyes on them. These companies are a lot more efficiently priced. You can do the legwork yourself- getting on management calls, reading industry news, poring over the financial statements to find strength or weakness that are not widely known.

So lets say you are a 10 billion dollar fund. You do your research and find that there is a small $100 million dollar company that doesn't have much debt, is growing rapidly, and the management seems good and there are good tailwinds for the industry as whole.

You have 100x the deployable capital than the company's market capitalization. Even if the stock price doubles or triples in price, your potential upside for the firm as whole is a relative drop in the bucket. You can't deploy more than a few million into this company without moving the market. For a large fund, it isn't worth wasting time chasing these small opportunities. In fact, "scaling" is a hard problem in funds.

Hence, there is actually a lot of opportunity out there for small scale investors who are willing to look under rocks that bigger guys don't see enough potential opportunity in.

Typically it means you can invest in niche markets. These market would be out of reach for larger players as they would struggle to find liquidity for their portfolio size. On the other end, smaller players would not be able to adjust their positions sufficiently granularly due to minimum lot/clip sizes specified on the market - similar to a quantisation effect if you don't sample at a sufficiently high frequency.

Many examples: faster in and out of the market, better fills, will not end up owning a company by mistake (e.g. bought all shares), etc... The bigger you get (in billions) the closer the returns will be to the market average...