So far, it's been a great year for ETFs and exchange-trade notes. Already these instruments have added more than $83 billion in assets, says IndexUniverse. That's a gain of nearly 5.5% in just four and a half months.

But some ETF gains stand out from even these gold-rush results.

It may be no surprise that the biggest ETF gainers so far this year seek to ride the coattails of the Nikkei's stunning growth. WisdomTree's Japan Hedged Equity fund has piled on $6.7 billion this year, nearly tripling in asset size. iShares' MSCI Japan ETF came next, adding $5.1 billion.

Among other big flow gatherers were U.S.-centric funds like iShares' Core S&P 500 (up $2.91 billion so far this year) and iShares' Russell 2000 (up $1.93 billion).

Vanguard's Short-Term Bond fund ($2.86 billion) leads among fixed-income issues, and the same manager's REIT fund is the most popular alternative.

One surprise: The eighth-biggest asset grabber among ETFs so far this year is iShares' MSCI USA Minimum Volatility product (up $2.51 billion) — suggesting that some people worry the U.S. stock rally could be vulnerable.