Presumably this is why Pu-238 is used for RTGs where power density matters; a half life of 87.7 years means a reasonably long working life using with only 1/3% of the mass compared to Pu-239.
Oh yeah you're probably right, I didn't think about it being a different isotope. I figured they just kept it near-critical to keep it toasty instead of a shorter half life.
