A deep dive into C++ memory allocation reveals hidden costs for React Native developers. The core finding is that `malloc()` rarely allocates the exact bytes requested; the operating system adds overhead for metadata and alignment. A simple test asking for 30 bytes resulted in 40 bytes being allocated on Linux, a 25% waste. This isn't just academic—it directly impacts the New Architecture and JSI communication.

This overhead becomes critical when passing large data sets between JavaScript and native code. Sending 100,000 small objects via JSI can waste megabytes of RAM. The lesson is to batch data into a single `ArrayBuffer` or string for one large allocation instead of many small ones. Platform differences also matter: iOS often rounds allocations more aggressively than Android, so testing on real devices is essential.

Developers should not panic, but understand these mechanics. For 95% of apps, Hermes and the JavaScript engine handle memory well. For the 5% with intensive native modules, prioritize contiguous data structures and align buffer sizes to powers of two (4096, 8192) to minimize waste. The goal is balancing performance with practical memory management, not obsessive byte-counting.