It seems that I cannot reproduce your issue. After changing the Width of dropdown control, the selection menu also changed its width, both on my pc and my mobile. What is the os version of your mobile? Please try to test with different ones.
Is it a standalone dropdown control or is it inside a form/gallery control? Also share the Width you used for the dropdown control.
Community Support Team _ Mona Li If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.