It is not just that.
There are additional rules that can limit a scrip from qualifying for an index,
or further limit the maximum weigtage of a scrip,
and other such additional limits
(justifiably?) to limit risk (eg. concentration risk).
Here are a few such for NIFTY 50.
Source: Rules/methodology of the NIFTY Equity Indices.
Also, on a somewhat related/tangential note, it is free-float market cap,
which is significantly different from market-cap, for certain securities.