Because the base year of sens*x is different from the base year of Nifty

SENSEX HAS SIX DIGIT WHILE NIFTY HAVE FIVE , SO SENSEX IS BIG

The base value of the SENSEX is taken as 100 on April 1, 1979, and its base year as 1978-79

while the base value of the Nifty is taken as 1000 on 1994, same1994 as base year.

No of companies in SESEX is 30 while in nifty the no of companies is 50.

so the market  cap differs in nifty and ses*x.
 

In both BSE and NSE has Diffrent Base Year as well as Diffrent Company as well as Diffrent Weightage to the Industry.

for Example Base Year is Diffrent in BSE Base Year is 1979 and In NSE base Year 1994, in sens*x there is 30 Company and in NSE there is 50 Company. but there is Correlation Between Both

Its not like sens*x have the greater value. The Only thing that makes the difference is base year. Infact nifty keeps more signifiance in terms of trading. Its havng 50 Blue chip stocks to form the index while sens*x is having only 30 bluchips.

In India, the Sens*x Nifty are the two benchmark indices. The Sens*x is the BSE’s main index, while the Nifty is the primary index of the NSE. However, if you look at the value of the two indices, you will see that the Sens*x is more than three times the Nifty. That is because the Sens*x is a significantly older index than the Nifty and has been calculated since 1978-79. In contrast, the Nifty base year is 1995, starting at a base value of 1000. 100 was the base value of the Sens*x, but by 1995 the Sens*x had crossed 3000 since the Indian stock market witnessed growth during those years. Consider reading the article on the difference between the Sens*x and Nifty on the IIFL Securities website.

For more information visit:- 

https://www.indiainfoline.com/knowledge-center/share-market/what-is-sens*x-bse-nse-nifty