Index Market Cap in US Market ::

List of index and the percent over total US stock market. source from wikipedia and the offical websites.

Index	Market Cap (%)	Comment
S&P 500	80%
S&P 400	7%
S&P 600	3%
S&P 1500	90%
S&P 100	51%
Russell 3000	98%
Russell 1000	88.2%	90% of Russell 3000
Russell 2000	9.8%	10% of Russell 3000
Russell Top 50	39.2%	40% of Russell 3000
Russell Top 200	68%	63% of Russell 1000 (wiki), 68% (official)
Russell MidCap	27.3%	31% of Russell 1000, 800 smallest companies in Russell 1000
Russell Microcap	< 3%	1000 smallest securities in Russell 2000

Table: Major index and their market cap.

Some numbers might be different from the offical website. For example, as of 2020-03-01, Russell Top 200 FactSheet says:

The index represents approximately 68% of the US market.

However, wiki say

Russell Top 200 represents 63% of total market capitalization in the Russell 1000 Index.