Why is my Legacy FileVault image larger than the physical disk?
According to this Apple Support discussion this is normal and is due to the mounted volume size being a "potential" size (usually twice) of the actual hard disk size. If it puts your mind at rest, I see exactly the same thing on my Snow Leopard 10.6.8 system with FileVault enabled and that's been just fine.

In addition, in my answer to FileVault sparsefile uses double of actual allocated disk space it could simply be that as Filevault mounts (and links to) the encrypted disk image, the size reported in Get Info is a result of the structure being traversed twice.