Carpet area reality is a rude shock

Visited Town Park Buildcon's project in Zeta II last week. Builder ka yeh super area ka chakkar toh I had no idea about. Brochure mein 1200 sqft dikhate hain, but actual carpet area toh 750 sqft bhi nahi nikla. Matlab, 40% se zyada toh shared area mein chala jaata hai! Ye builders itna difference kaise justify karte hain? My parents bolte hain this is common, but for first-timers like me, it feels like a big cheat. Is there any way to negotiate this, or should I just walk away?