Be a bit more conservative with allocating memory

Cap the max chunk size at 2MB. This value still still doesn't
affect the v8 benchmark noticably, but should avoid extreme
memory usage in some corner cases

Task-number: QTBUG-37134
Change-Id: If2050374c4a7df7ff74194d64880e2d660ea26fd
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>