Mac OS 10.14.1
Minecraft versions with the new splash screen take decently more RAM on launch compared to snapshots without the new splash, and at least twice as much RAM compared to 1.13. This RAM usage occurs without loading a world and before the main menu, so it may be because of either the loading bar or the new resource-pack-compatible logo.
I ran a couple tests to get numbers, but these might not be accurate to all machines. They were run using the profiles for latest snapshot, latest release, and latest snapshot with the version changed to 05a.
19w08b: ~2.4-2.5 GB RAM
19w05a: ~2-2.1 GB RAM
1.13.2: ~1.1-1.2 GB RAM