Reliability of the results

We recently conducted a research for one of our client's ad effectiveness campaign. We had asked questions to a control group who were not exposed to the ad and the same questions to the test group that was exposed to it.

Our standard practice it to collect at least 300 responses. We had N=321 for the Control and 387 for the test group. The concern was the difference in number of respondents between the two groups (387-321) = 66.

Does anyone know if this difference will skew the results too much? We are providing results at 95% confidence level.

Any insight is highly appreciated.