I have an account at Fidelity where I purchase ETFs every few weeks with contributions from myself and my employer. I have been buying ITOT, IXUS, and AGG (total stock market, total international stock market, total bond market). They have low expense ratios and trade commission free at Fidelity. However, I have noticed the market cap of ITOT and IXUS is pretty low compared to, say, Vanguard's total stock market ETF of VTI (ITOT: $540M versus VTI: $27B), presumably because they are fairly new--both less than a year old. So I was wondering, what are the risks regarding ETFs with relatively low market caps?
Market cap probably isn't as big of an issue as the bid/ask spread and the liquidity, although they tend to be related. The spread is likely to be wider on lesser traded ETF funds we are talking about pennies, likely not an issue unless you are trading in and out frequently. The expense ratios will also tend to be slightly higher again not a huge issue but it might be a consideration.
You are unlikely to make up the cost of paying the commission to buy into a larger ETF any time soon though.