U.S. stocks have been in rally mode for most of this year and looked poised to end 2013 in strong fashion. Still, the market is getting smaller on multiple fronts.

Not only are there far few companies trading in the U.S. today than were in the go-go days of the 1990s, but companies are buying back their own shares at a voracious clip. “The total number of U.S. exchange-listed companies peaked near 8,800 in 1997 and has since sunk to 4,900 as of year-end 2012,” reports Michael Santoli for Yahoo Finance, citing data from Strategas Group.

Delistings of failed technology companies following the bursting of the tech bubble, a raft of mergers and acquisitions prior to the financial crisis and several years of slack initial public offering activity are among the factors contributing to few listings, notes Santoli. Buybacks are playing a part as well and one that is benefiting some ETFs.

FactSet reports that S&P 500 companies repurchased an aggregate $415 billion the past four quarters, up 12% from a year earlier, according to Yahoo Finance. Increased buyback ETF has benefited the PowerShares Buyback Achievers Portfolio (NYSEArca: PKW), among other ETFs. Year-to-date, PKW is up 40.4% compared to a 29% gain for the S&P 500. [Buyback ETF Still Soaring Despite Tapering Chatter]

PKW is impressive on several other fronts. The ETF has surged despite not holding Apple (NadsaqGM: AAPL), one of the most voracious buyers of its own shares. Although Apple has been buying back its own shares and may do more of that if Carl Icahn gets his way, it has not yet qualified for admission into the NASDAQ US Buyback AchieversIndex, PKW’s underlying index.

The NASDAQ US Buyback Achievers Index stipulates that constituent companies have effected a net reduction in shares outstanding of 5% or more in the trailing 12 months, according to PowerShares.