If you want to bring contrast to your typeface where do you get the contrast in Latin? It's from the tool that writes the script. It could be a broad nib pen or expansion pen. The angle of the pen causes the vertical strokes become thicker in Latin. This naturally happened because the most important part of the construction in Latin is in vertical strokes. If you make the the verticals thinner it would become much harder to recognize the shapes in text sizes. Actually Latin calligraphy starts with practicing lots of vertical strokes.
In Arabic the angle of the pen causes the contrast to be more horizontally stressed and this happened naturally because the construction of arabic is majorly distributed horizontally or in simpler words Arabic script is mostly horizontal strokes. If you make the horizontal strokes thinner in Arabic then it is Latinized or reversed contrast.