U.S. Small Caps are indeed in focus again as we alluded to on Monday in this column, as we saw several headlines yesterday about fund manager Doug Kass recently establishing and adding to a position in TZA (Direxion Daily Small Cap Bear 3X Shares, Expense Ratio 0.95%).

But today we would like to focus squarely on U.S. Mid-Caps, as there has been considerably heavy trading action in the space recently.

First off, last week we saw late month ETF model changes that rippled through two prominent Mid Cap equity ETFs IJH (iShares Core S&P Mid Cap, Expense Ratio 0.15%) and MVV (ProShares Ultra MidCap400, Expense Ratio 0.95%), with both funds seeing inflows in the $400-$600 realm.

Now in recent sessions we are seeing good sized outflows in related ETF MDY (SPDR S&P MidCap400, Expense Ratio 0.25%), to the tune of >$700 million. The fund is an old veteran of sorts, having debuted in April of 1995 when Montell Jordan’s “This Is How We Do It” was in heavy play, unfortunately, on the radio at the time.

MDY also has greater than $15 billion in assets under management, so we will not read into the outflows too much yet. IJH debuted five years after MDY in 2000, and has since grown into the largest U.S. Mid Cap Equity focused ETF, surpassing MDY with about $23.2 billion currently in AUM.

Interestingly, the third largest Mid Cap concentrated ETF in the U.S. marketplace tracks a Russell Index, not an S&P one as do IJH and MDY, and this fund, IWR (iShares Russell Mid Cap, Expense Ratio 0.22%) has been receiving some attention via trading volume in the marketplace lately as well and is now a $10 billion fund. Both the S&P and Russell Mid Cap Indexes are market cap-weighted, but neither has what we would call a heavy concentration in any particular security.