Object sizes in JRuby are certainly larger, especially if you run on a 64-bit JVM which necessarily has 64-bit reference fields for all object references (basic

Object sizes in JRuby are certainly larger, especially if you run on a 64-bit JVM which necessarily has 64-bit reference fields for all object references (basically everything in Ruby). It doesn't surprise me at all to see a single-threaded case with a lot of objects use more memory, but it would be interesting to see how much of that was unused heap space and how much was actually live data. MRI's less-efficient conservative collector can live in a smaller memory space, but you sacrifice performance.