You probably got 32 bit Java (Oracle loves to do that, for some reason) and your designer is set to launch with more than 1.5gB of RAM, so 32-bit java is unable to allocate it.
You can check Configure → Gateway Settings and reduce the max designer memory, or check your installed Java architecture (if Windows, then just searching for “Java” should give you ‘Configure Java’, which will bring up the Java control panel and let you check architecture):
