Teach me about shoe trees

I bought several pairs of Woodlore Ultra trees in what I'm assuming is the correct size; the size recommended by Woodlore corresponds to the size of the shoes. When I insert the shoe tree, it fills the upper, puts quite a bit of tension on the vamp and straightens out the sole. I imagine this will help alleviate creases, but I'm concerned the tree is too big for the shoe. The tree is bigger than my foot.

Please pardon my ignorance, but I appreciate your thoughts.