I just notice that Lightning:buttonMenu aura component size bigger in the mobile after winter 20 Salesforce release.
Is it any way to control the size as previous because it's breaking our UI. Its working fine in LXP and Community.
This is by design and purposely done for A11Y reasons (Touch targets should have a minimum target size of 44 pixels (px) wide 44 pixels tall).
You'd have to overwrite the styles for mobile.