as in some series in past year hdfc ltd had 250/ lot and in some series it had 500/ lot like this many other too did
Its simple, to make F&O a lot more affordable.
Between 2 Lacs to 4 Lacs all the F&O lots are available. So if ITC is trading at 360 with lot size 1000 and after a year if it trades at 600 automatically lot size will be reduced to 750 or 500 to make it affordable.
Here is a SEBI cirular relased in 2010 directing excahnges to fix lot sizes.
Hoe this helps.
SEBI fixed Rs 2 lacs (Approx.) for Lot value for F&O Stock, if the price value increase or decrease of Stock, Qty in one lot will Decrease and Increase by exchange.