I think market cap is one of the criteria for admission into the...

I think market cap is one of the criteria for admission into the S&P/ASX20, or S&P/ASX200 or S&P/ASX300. Other requirements are liquidity, ASX listed etc..

Institutions do balance their portfolios when shares are included in/excluded from the indices.

Whilst $100m may not be the magic number or the threshold to trigger institutions buying, I am quite confident that BIG will be on their radars if its market cap goes north with the prospect of index inclusion.