Step 1: Calculate the number of male employees in company 'D'. 
Total employees in D = 2500 
Female employees in D = 1220 
Therefore, male employees in D = \(2500 - 1220 = 1280\) 
Step 2: Find the number of female employees in company 'C'. 
Female employees in C = 1600 (given) 
Step 3: Calculate the difference in numbers. 
Difference = Female employees in C - Male employees in D 
\[
= 1600 - 1280 = 320
\] 
Step 4: Calculate the percentage by which the male employees in D are less than female employees in C. 
\[
\text{Percentage less} = \frac{\text{Difference}}{\text{Female employees in C}} \times 100 = \frac{320}{1600} \times 100 = 20\%
\] 
Thus, the number of male employees in company 'D' is 20% less than the number of female employees in company 'C'. Hence, the answer is \(\boxed{20\%}\).