The weightage of constituents in NIFTY index is documented every now and then by NSE on their website. Here is the link. Dropdown 'Method of Computation' and download the file 'Download Industry-wise Market Capitalisation, Weightage, Beta etc. (.csv)'.
With the fall of the major constituent stocks Nifty falls, since Nifty falls, traders who trade in the other constituent stocks also pull the stocks down, pulling down Nifty further.
It doesnt mean if ITC which is a major weightage falls (Nifty falls), other stocks with minor weightage won’t fall. Always when breakout occurs, it cascades to other stocks also. So you should be careful on this front.