Do the Brokerage Charges Differ for different Lots of different Companies?

#1
Can anyone let me know why brokerage charges are different for different Company Future Lots?
Ex:
a) Minifty - 20 shares:1 Lot ( Rs.20.00 + Rs.20.00 Per Buy/Sell Transaction )
b) Nifty - 50 shares: 1 Lot ( Rs.50.00 + Rs.50.00 Per Buy/Sell Transaction )

How can i find out about how is charged for other Future Lots like Reliance,Cairn Etc.

Thanks in advance.