I have been asked many times the above question. I can only answer the question indirectly. If an ETF fund size is low, it becomes unprofitable for the fund manager. Naturally it will close down eventually. An example is XShares Advisors which will be liquidating its Adelante  family of ETFs. I just chceked that the liquidity is very poor on their ETFs. While low liquidity does not automatically imply low fund size, the low liquidity is an indication of the lack of interest by investors to invest in a fund. Read here: HERE for more information on Adelante ETFs.