Playing with 41 army-sizes I did increase the values by 1.5x in supply_system_values and it works great.
low_fertile_regions = 30,
low_fertile_regional_cap = 250,
normal_regions = 38,
normal_regional_cap = 300,
fertile_regions = 45,
fertile_regional_cap = 350,
very_fertile_regions = 53,
very_fertile_regional_cap = 400,