Mini index contracts exist to bridge the gap between a $500,000-notional spx-options contract and a much smaller ETF option. The multiplier is the same $100, but the index level is one tenth, so notional is roughly $50,000 per contract.
They keep the features that matter — cash-settled-option settlement, european-style-option exercise, index tax treatment — while letting an account of $25,000 build a proper iron-condor or collar without rounding to zero or one contract.
Example: an index at 5,000 has a mini at 500. A 10-point-wide mini vertical-spread risks $1,000 per contract instead of the $10,000 the full-size version would risk. A trader with a $500 maximum loss can trade half a spread's worth of structure rather than skipping the trade.
Related: spx-options, index-option, etf-option, cash-settled-option