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 \(\boxed{20\%}\).