The 41 mm thickness is the same as the 38 mm

Notice how the bracelet for the 42 mm can break the lines from the case. The 41 mm retains end-link to case integrity and flow. 
38 mm
41 mm

Dial size of the 41 mm. Dial size if the 38 mm is 27 mm
Dial size of the 42 mm. Same as the 41 mm

See the perfect “flow” of the case to bracelet for the 41 mm
The bracelet of the 42 mm is different at the end link and does not integrate as well to the case. The 41 mm integrates better. The 41 mm also has tighter tolerances.
GP changed the bracelet supplied for the 42 mm because of cost reasons. 