Are you actually hibernating it, or doing the "hybrid sleep" thing, which only sleeps and then hibernates? In my case (Arch) the default for hybrid sleep is to keep it in sleep state for quite a while, whereas windows shuts down fairly quickly when unplugged.
Actual hibernation is shutting the computer completely off, so I'd be curious to know how one OS could have a different power consumption than another.
The way I found out is this one time I left work with my laptop under windows and asleep.When I arrived home an hour later, it had to go through the full bios setup (which you can't miss since it's ridiculously slow) and then it went and booted Linux (it's the default boot setup). When I rebooted back to windows, I found my previous stuff open. Since this happened again a few times, I figured it was normal behavior.
My default Linux install won't automatically hibernate while asleep as long as the battery is above some threshold, like 10%. And since the computer doesn't support S3, only "modern standby", it will chew through the battery like no tomorrow.
To diagnose hibernation issues, I'd double check the kernel command line (resume=<disk or inode>) argument. That it's still the right location.