Wall Sina has a radius of ~250km. The distance between Wall Rose and Wall Sina is ~130km, so it has a radius of ~380km. The distance between Wall Rose and Wall Maria is ~100km, so it has a radius of ~480km.
The inclusive areas of Walls Sina, Rose, and Maria are therefore ~200,000; ~450,000; and ~720,000 km^2 respectively.
The exclusive areas of Walls Sina, Rose, and Maria are therefore ~200,000; ~260,000; and ~270,000 km^2 respectively.
The percentage of the total area that the exclusive areas of Walls Sina, Rose, and Maria represent are therefore 27%, 36%, and 37%.
Conclusion, losing the area exclusive to Wall Maria does in fact represent a loss of about 1/3 (33%) of humanity's territory.
To reiterate the comparison, the total inclusive area of Wall Maria (~720,000 km^2) represents ~1.3x the area of France, 2.0x the area of Germany, 3.0x the area of the United Kingdom, and is basically equal to the size of Texas.