hi umar syed
I think the total gain should be the same, if the total input powers are the same for both cases.
Did the power of your single carrier match the total power of your multiple carriers combined?
If not, you may have seen gain saturation…
Did you do simulations or experiments?
If simulations, are all your carriers modeled as CW sources (without linewidth), or did you consider them to have a certain bandwidth?
Hope that helps somewhat, with reagards