Let's analyze the relationship between the letter groups: 
- CAT : DDY 
Look at the positions of letters in the alphabet: 
C (3), A (1), T (20) 
D (4), D (4), Y (25) 
Difference between each corresponding letter: 
3 → 4 (+1), 1 → 4 (+3), 20 → 25 (+5) 
So, the increments are +1, +3, +5 respectively. 
Apply the same to BIG: 
B (2), I (9), G (7) 
Adding +1, +3, +5 respectively: 
2 + 1 = 3 (C), 9 + 3 = 12 (L), 7 + 5 = 12 (L) 
Therefore, the answer is CLL.