It was replaced by the KitKat Extra Crispy in 2006.

In 2016, the "BigKat" name was re-introduced, but the product is still significantly smaller than it once was.

Photos from 2006.

Net weight
Original (2006) BigKat 1.94oz 55g
KitKat Extra Crispy 1.61oz 45g
New (2016) BigKat 1.5oz 42g

Calories
Orig BK 290
KKEC 240
New BK 210

Fat
Orig BK 15g
KKEC 12g
New BK 11g

Carb
Orig BK 35g
KKEC 29g
New BK 27g

Protein
Orig BK 3g
KKEC 3g
New BK 3g

2006:
Manufactured by H.B. Reese Candy Co, a division of Hershey Foods Corporation, under license from Societe des Produits Nestle S.A.

2016:
Distributed by The Hershey Company Under License from SdP Nestle SA