I shouldn't be surprised, but my knowledge of the ruby world seems to be about two years out of date. I suppose any remaining additional memory usage is probably due to inheriting space/speed tradeoffs straight from upstream hotspot.
It's great to hear that the startup time has improved so much.
Actually a lot of the process size in JRuby is the fact that we need to set JVM's max size high for rare cases, but then the JVM happily grows to fill much of that size. If you choke it down to a smaller size, we're competitive with at least generational-GC impls like Rubinius (but still much larger than conservative-GC impls like MRI).
It's great to hear that the startup time has improved so much.