Get the biggest number from a two dimensional list

[rs74]

This is what I get (same result both rows, biggest should only show the largest number):

list: [[3543, 983], [2782, 1863], [4784, 2178], [3676, 2152], [3643, 5325], [4830, 362], [5752, 3868], [1551, 4228], [4135, 6600]]
biggest: [[3543, 983], [2782, 1863], [4784, 2178], [3676, 2152], [3643, 5325], [4830, 362], [5752, 3868], [1551, 4228], [4135, 6600]]

import random
 
def biggest(li):
 
    for a in m:
        max(a)
 
    for b in m:
        max(b)
 
        return m
 
 
m = []
 
a = random.randint(1,5)
 
b = random.randint(5,9)
 
for i in range(b):
 
   l = []
 
   for j in range(a):
 
     l.append(random.randint(-500, 7000))
 
   m.append(l)
     
print ("list:", m)
 
print ("biggest: ", biggest(m))