$标普500指数 (.SPX.US)$ $景顺S&P小盘能量投资组合 (PSCE.US)$ $Invesco S&P SmallCap Health Care ETF (PSCH.US)$ $The spreads tightening thing is interesting, though I suspect is more correlation than than causation as more of an example of risk on vs risk off.  You can also point to things like Copper or the Yen as also occurring at times of Small cap outperformance, but that's not likely a cause, more like just a correlation.  As implied the low high yield spreads are likely to keep a lid on any major upside, although, you could certainly see 10-20% in equities with a little bit more tightening.  What is interesting to me is high yield has yet to participate in new highs yet.
healthcare has been a drag on the index, the only sector with negative contribution, so you are probably correct in suspecting healthcare needs to do some lifting here.
AMC's 1600% gain this year has also lifted IWM the most of any other holding, adding 68 basis points of return.  It's only 0.6% of the index, but is the largest holding, and is 2x as big as the next largest holding, so it has mattered a tiny bit.
It's amazing that all of small caps gains this year basically came within the first 45 days of the year, especially when you compare this to large caps, which continue to be lifted by the top 7 megatechs and have added another 10% to gains since April...