When you want a Splitter to take size in percentages, you have to make sure that all of its parent also have size configured in percentages (up to a parent with a fixed size). Do you experience the same issue when you set a fixed hight to the Splitter's wrapping div? If no, most probably there is any parent which size is not configured. This is not anything specific for the Splitter, but is a general HTML behavior.
I would suggest that you review the online resources where you ca find detailed explanation on how to configure a Splitter properly:
Filling the Entire Page
Resize with Window
If you still experience the described issue after applying the configurations from the articles above, I will need to examine you markup in order to see what is causing the issue. You can see how to prepare a sample fully runnable project in the following blog post: Isolating a problem in a sample project
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