Your motherboard (H61 chipset) supports up to 16GB DDR3-1066 or DDR3-1333 modules with a max configuration of 2 x 8GB to get to 16GB.
You have DDR3-1600 modules which are scaling down to DDR3-1333 (the max your board supports). Having non-matching sized 2GB and 4GB modules will have you running in single channel mode (reduced performance) while your motherboard is capable of dual channel mode. Always install modules in matching pairs to take advantage of the dual memory controllers. Your options are to remove the 2GB module, then install a matching 4GB DDR3-1600 CL9 module (2 x 4GB for 8GB in dual channel mode), or remove both modules and install a matching pair of 8GB modules (2 x 8GB DDR3-1333 or DDR3-1600).