To reproduce:

1. Download attached project "MemoryUsage.zip" and open in Unity
2. Open "SampleScene" scene
3. Enter Play mode
4. Observe that Memory usage is within 200 MB
5. Exit Play mode
6. Change Scripting Runtime Version to ".NET 3.5 Equivalent" and Scripting Backend to "Mono"
7. Set Target Platform to "Windows" and Architecture to "x86_64"
8. Build And Run
9. Observe that Memory usage is within 200 MB
10. Set Architecture to "x86"
11. Build And Run
12. Observe that Memory usage is within 200 MB
13. Change Scripting Runtime Version to ".NET 4.x Equivalent"
14. Set Architecture to "x86_64"
15. Build And Run
16. Observe that Memory usage is within 200 MB
17. Set Architecture to "x86"
18. Build And Run

Expected result: You will see that Memory usage is still within 200 MB
Actual result: You will see that Memory usage allocates up to 3GB until it produces "out of memory" exceptions

Notes:
- This issue reproduces only on Windows
- If you set Scripting Backend to "IL2CPP" and Architecture to "x86", the build will crash

Reproduced on Unity 2017.1.4p2, 2017.2.3p2, 2017.3.2f1, 2017.4.6f1, 2018.1.7f1, 2018.2.0b11 and 2018.3.0a3