Here's a quote from a very helpful article I found:
"It is advisable to use RAM sticks by the same manufacturer, of the same size, and of the same frequency. But there's a simple reason behind why mixing RAM sizes is usually not the best way. RAM has several components that all come together to make it perform well.
For two different size RAM sticks to perform optimally together, they need to use the same voltage and their respective controllers should play well with each other and the motherboard. That's why it's best to use the same model in all slots.
However, this doesn't mean you can't use different size RAM sticks together. For example, if your first stick is 4GB, you can still add a new 8GB stick. Once you switch on dual channel mode (also called flex mode), it will perform as two 4GB sticks running side by side in optimal performance.
The remaining 4GB of the new stick will run in single channel mode. Overall, it's not as fast as using two sticks of the same size, but it's still faster than what you had before.
It's the same with frequency or speed. Your RAM sticks will work together at the frequency of the lower stick, by default. So do RAM sticks have to match? No, but it's better if they do."