Here's a shot of the custom 18w HX-NPF LPS ballast that's in my Philips Goldeye. It's a Philips ballast that's elongated to fit in the Goldeye's ballast compartment, it's around the size of a NPF F40/RS fluorescent ballast.
I guess it's because the Goldeye is pretty flat compared to a wallpack...one thing's for sure though finding a replacement ballast for this won't be easy.