When viewing leaves that have a value of 18:4 or higher or 161:2 or higher are not textured in game. At this point they may not be naturally spawning but they can be given still. In addition players can still have this item on them or in chests.
When holding these blocks the items appear much bigger than they normally should. All checks done in vanilla.
You can't obtain them without the use of commands. Works as intended.