Ruby is not necessarily more resource hogging than PHP, but the way typical ruby webapps deployed (all workers processes started immediately and left idling to wait for requests) leads to higher memory usage compared to low traffic PHP webapp (worker processes only started when requests is coming and shutdown afterwards, so you can host a lot of small traffics sites in a small server). If PHP webapps are deployed with similar process lifecycle, I imagine it would consume similar resources.

For medium and high traffic webapps, the difference is moot because you're going to end up with a large amount of worker processes to handle all those traffics anyway regardless of how you initially spawn them.