Steam starts 32bit by default -.- googled how to change the launch options, hope it works now, thx.
Edit: Many hours later, it still works. KSP is taking 4.4 GB of ram now, so it apparently ran out of memory. For those using Steam, the command is:
"E:\Steam\steamapps\common\Kerbal Space Program\KSP_x64.exe" %command%
Of course, you have to enter the correct system path on your system.