To reproduce:
1. Open users attached project "MeshCompression 2018.2.20.zip"
2. In Player Settings make sure Vertex Compression is enabled on everything except Position and Color and that Optimize Mesh Data is enabled
3. Select Assets > WR_ThroneRoom > WR_ThroneRoom_01 and make sure that Mesh Compression is off and Read/Write Enabled is disabled
4. Build for iOS with Development Build and Autoconnect Profiler enabled
5. Deploy the Xcode project to the device
6. In Unity open Profiler window
7. Select the memory tab and select the readout from Simple to Detailed
8. Press "Take Sample" in the Profiler window
9. In the Profiler window check Assets > Mesh > WR_Throneroom and observe the memory used

Expected result: memory usage is the same on 2018.3 version as in 2018.2
Actual result: on 2018.3 mesh uses more memory than in 2018.2 even though the settings are the same

Results:
2017.4.21f1
89.0KB

2018.2.20f1
89.0KB

2018.3.0a1
89.0KB

2018.3.0a2
111.9KB

2018.3.0b1
112.0KB

2018.3.7f1
112.0KB

2019.1.0b5
112.1KB

2019.2.0a6
112.0KB

Devices tested on:
iPhone 8 Plus iOS 11.0

Notes:
On Android, the memory usage increase is really small from 88.7kb to 88.8kb