The bug

Since 1.13, the RAM used by the game has increased drastically to reach more than 5 GB in 1.15.2 and higher when Minecraft is not limited by JVM default arguments (= put -Xmx8G for example)!

For comparison, 1.12.2 uses only 500 MB of RAM!

This problem is apparently due to the DataFixerUpper introduced in 1.13.

How to test

1. Go to the "Installations" section of your Minecraft launcher.
2. Click on the three dots that appear when you hover over the profile you want to use for this test, and then click "Edit". If you don't already have a profile, create one!
   

1. Select the version you want to test.
2. Click on "More Options"
3. Change the "-Xmx" argument in the "JVM Arguments" section to allocate 6 GB of RAM or more. Example for 8 GB (recommended) : -Xmx8G
4. Save your profile.
5. Open the task manager of your operating system.
6. Launch the game with the correct profile and observe the evolution of the RAM used by the game in the task manager.
7. Repeat for each version you want to test.
   

Videos tests

Here are all the tests I did from 1.12.2 to 1.16 Pre-Release 2, on all stable versions of Minecraft.
I have allocated 8 GB of RAM each time so as not to restrict the game to too low a ceiling, which would not allow us to see the problem.
No other programs run on the PC (except Discord and my recording program).
I did the test on my two different machines and the results are the same.

• Minecraft 1.12.2 (~450 MB) : Client RAM Test 1.12.2.mp4
• Minecraft 1.13.2 (~2.1 GB) : Client RAM Test 1.13.2.mp4
• Minecraft 1.14.4 (~5.0 GB) : Client RAM Test 1.14.4.mp4
• Minecraft 1.15.2 (~5.2 GB) : Client RAM Test 1.15.2.mp4
• Minecraft 1.16 Pre-Release 2 (~5.2 GB) : Client RAM Test 1.16 Pre-2.mp4

We can also observe that with each new version, the CPU is used harder and harder, even after the game is fully loaded (especially in 1.16).

Note: This ticket is attached to MC-190258 which processes the server side.