Assuming equal probability of the deck where the split is.
I can find the expected size of the smaller deck of card is $\frac{1+n}{2}$, but I am not sure how the find the expected ratio in a closed form of $n$.
Fundamentally, it would be something like $\frac{1}{n}\sum_{i=1}^n \frac{i}{2n+1-i}$. Not sure if there is a closed form. Maybe there is a different approach.