Whenever I run a modpack, the game crashes and I go back to the launcher.

[Sirfurnace]

Whenever I launch a game from the launcher the game crashes and it goes back to the launcher. I have tried this with B-Team, Tekkit, and a custom modpack I made for myself. When it crashes, the console says:

[B#347]  Error occurred during initialization of VM
[B#347]  Failed to allocate initial heap.
[B#347] Analytics Response [runModpack]: 200

I don't know what this means. I'm running the latest 64-bit version of java and I have 15 gigabytes of data allocated. I am running minecraft 1.7.10. What's happening?

[AetherPirate]

Don't allocate more than 4GB of ram. 

and pastebin a log.