RTY is the small-cap leg of the index complex and the standard instrument for expressing a view on domestic, smaller, more rate-sensitive companies against the large-cap es. It is materially more volatile than the S&P, so equal contract counts are not equal risk.
It is also the usual cross-hedge for a small-cap equity book, and the short leg of the widely traded large-cap-versus-small-cap intercommodity-spread. The annual Russell reconstitution in June produces enormous volume as index funds rebalance.
Example: index at 2,240, one contract is 2,240 x $50 = $112,000 notional. A 1.5% day is 33.6 points, or $1,680 per contract, against about $950 on one ES contract in a 1.0% S&P day.
Related: es, equity-index-futures, intercommodity-spread, cross-hedge, micro-futures