Hi,
1. Check if you add any page break for the first tablix.
2. In SSRS 2008 or higher, there is a Report Property ConsumeContainerWhitespace which controls whether white space in containers should be consumed when contents grow instead of preserving the minimum white space between the contents and the container. The value is set to False by default. So in your scenario, if you are sure there is no extra blank space in your report, please set value of ConsumeContainerWhightspace to True.