Yes, depending on your definition of big.
In the article “Mortgage-REIT ETF is big bet on one stock”, the MarketWatch reporter John Spence points out that iShares FTSE NAREIT Mortgage REITs (REM) had 29% of the fund invested in one stock, Annaly Capital Management (NLY).
If ETFs are supposed to deliver instant diversification, how did that happen? It turns out that SEC rules require domestic ETFs to hold only a minimum of 13 securities. In addition, a fund cannot invest more than 30% of the fund in any one security or more than 65% in the 5 most-heavily weighted securities.
For international ETFs, the rules are stricter. A fund must hold a minimum of 20 securities with a limit of 25% in one and 60% in the top 5. For more background, see Pros and Cons of ETFs.
The good news is that it is easy to check if an ETF is heavily invested in one or two stocks as holdings are reported throughout the day. Good luck figuring that out for a mutual fund which only report holdings a few times a year and even that information is old by the time you get it.