12 Volt vs a 24 Volt Motor
The simplest answer is that the 24V motor would have twice as many turns of wire than the 12V motor (with thinner wire to fit the same armature). So even though the current is halved, the magnetic field is the same.

Below is from http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html

24V would have 1/2 of current I, but number of turns, n, is doubled, so B stays the same.