Even if using non-compound indexes, indexes on 1 field = seems like every index entry stores full names of fields. In our test cases field names if very long and deep (many levels of supobjects).

I think index may have name, ONE fields desription, order, and many entries with ONLY values, or values + field position.

More space index occupy - less total objects count fits in RAM (even big RAM).
and more space index occupy - indexes searches is slower.

Compact indexes - happy customers.