Hmm.. maybe it has to do with the Scenery it's loading?
Airports like EHAM (freeware), KIAG (payware), EGLL (Aerosoft with some additions) make X-Plane 10 easily reach 2.8 GB RAM of real memory.
Could also be the plugins...?
Do you have much scenery installed and/or use many plugins?
EDIT: Could you post a screenshot of the memory it uses?
I don't currently have any plugins. The only scenery enhancement I've used is the HD Scenery Mesh. I'll have to check when I get home. 64-bitness will be a very welcome improvement though.
http://www.alpilotx.net/downloads/x-plane-10-hd-scenery-mesh/