[MC-72906] Customized world with changed gravel settings causes java crash!?

I am not sure, if this is a problem with java, or with Minecraft, but when I created a customized world with the gravel preset of
Min. Heigth: 0
Max. Height: 50
Spawn tries: 40
SpawnSize: 50

This happens:

[14:49:18] [Server thread/INFO]: Starting integrated minecraft server version 1.8
[14:49:18] [Server thread/INFO]: Generating keypair
[14:49:18] [Server thread/INFO]: Converting map!
[14:49:18] [Server thread/INFO]: Scanning folders...
[14:49:18] [Server thread/INFO]: Total conversion count is 0
[14:49:18] [Server thread/INFO]: Preparing start region for level 0
[14:49:19] [Server thread/INFO]: Preparing spawn area: 10%
[14:49:21] [Server thread/INFO]: Preparing spawn area: 16%
[14:49:22] [Server thread/INFO]: Preparing spawn area: 22%
[14:49:23] [Server thread/INFO]: Preparing spawn area: 29%
[14:49:24] [Server thread/INFO]: Preparing spawn area: 36%
[14:49:25] [Server thread/INFO]: Preparing spawn area: 41%
[14:49:26] [Server thread/INFO]: Preparing spawn area: 48%
[14:49:27] [Server thread/INFO]: Preparing spawn area: 54%
[14:49:28] [Server thread/INFO]: Preparing spawn area: 59%
[14:49:29] [Server thread/INFO]: Preparing spawn area: 65%
[14:49:30] [Server thread/INFO]: Preparing spawn area: 70%
[14:49:35] [Server thread/INFO]: Preparing spawn area: 75%
[14:49:40] [Server thread/INFO]: Preparing spawn area: 78%
[14:49:44] [Server thread/INFO]: Preparing spawn area: 82%
[14:49:48] [Server thread/INFO]: Preparing spawn area: 85%
[14:49:53] [Server thread/INFO]: Preparing spawn area: 88%
[14:49:57] [Server thread/INFO]: Preparing spawn area: 90%
[14:50:02] [Server thread/INFO]: Preparing spawn area: 91%
[14:50:06] [Server thread/INFO]: Preparing spawn area: 93%
[14:50:11] [Server thread/INFO]: Preparing spawn area: 94%
[14:50:16] [Server thread/INFO]: Preparing spawn area: 95%
[14:50:20] [Server thread/INFO]: Preparing spawn area: 95%
[14:50:25] [Server thread/INFO]: Preparing spawn area: 96%
[14:50:29] [Server thread/INFO]: Preparing spawn area: 96%
[14:50:33] [Server thread/INFO]: Preparing spawn area: 97%
[14:50:37] [Server thread/INFO]: Preparing spawn area: 97%
[14:50:41] [Server thread/INFO]: Preparing spawn area: 97%
[14:50:45] [Server thread/INFO]: Preparing spawn area: 97%
[14:50:54] [Server thread/INFO]: Preparing spawn area: 97%
[14:51:03] [Server thread/INFO]: Preparing spawn area: 98%
[14:51:26] [Server thread/INFO]: Preparing spawn area: 98%
Exception in thread "Snooper Timer" java.lang.OutOfMemoryError: Java heap space
Exception in thread "Server thread" java.lang.OutOfMemoryError: Java heap space
Exception in thread "Netty Server IO #5" java.lang.OutOfMemoryError: Java heap space
Exception in thread "Netty Server IO #1" java.lang.OutOfMemoryError: Java heap space
Exception in thread "Netty Server IO #0" java.lang.OutOfMemoryError: Java heap space
Exception in thread "Netty Local Client IO #2" java.lang.OutOfMemoryError: Java heap space
Exception in thread "Netty Server IO #2" java.lang.OutOfMemoryError: Java heap space

I have this as java settings (so I didn't changed anything in the java settings):

-Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M

Also I had for some time periods, while Minecraft wasn't responding, two Minecraft Windows open (no I started Minecraft only once...), but one of them was only a "blank" java window which also didn't reponsed.

I could only reproduce this one time...

Even if you say this is invalid, etc. I say this happened and yes my Minecraft is unmodded!