Is there any option to change buttons padding in plasma 5.12 or QtCurve? I searched but didn't find any up-to-date useful reference.

There is a checkbox on the General tab of the QtCurve configuration for toggling Thinner Buttons which reduces the vertical padding.
I haven't seen anything for changing the padding in Breeze.
With Kvantum, the padding and such is defined by the theme and the SVGs used for the buttons.