SystemInfo.systemMemorySize uses sysctl to get Physical Memory size on iOS but sysctl has not been working since iOS 9 due to Privacy changes. The fallback heuristic method sometimes returns inaccurate values for Physical Memory. Using NSProcessInfo.physicalMemory might be a better way to get Physical Memory size.

To reproduce:

1. Download attached "ios-system-memory-size.zip" project and open in Unity
2. Open "Main" scene
3. Build for iOS
4. Deploy the Xcode project to the iOS device
5. SystemInfo.systemMemorySize will be displayed in UI Text
6. Observe that in certain conditions, SystemInfo.systemMemorySize will be under-report available Device Memory

Notes:
- This is a known problem with the implementation of SystemInfo.systemMemorySize on iOS

Tested with:
- iPhone 5c, iOS: 9.3.2 - Always 1015 Physical Memory
- iPod Touch 5th Gen, iOS: 9.3.5 - Always 503 Physical Memory
- iPhone 6s Plus, iOS: 10.3.2 - Always 1985 Physical Memory
- iPhone 7, iOS: 10.2 - Always 2000 Physical Memory
- iPad Mini 4, iOS: 11.4.1 - Always 1988 Physical Memory
- iPhone 8 Plus, iOS: 12.0 - Always 2990 Physical Memory
- iPhone XR, iOS: 12.0 - Always 2813 Physical Memory
- iPad Pro, iOS: 12.1 - Always 3769 Physical Memory

Reproduced on Unity 2017.4.0f1, 2017.4.27f1, 2018.3.14f1, 2018.4.1f1, 2019.1.4f1, 2019.2.0b3 and 2019.3.0a4