Apple TV have 40GB hard drive size??

Its running OS x which takes a few gigs + I believe there's a hidden backup partition to recover from so that doubles it

Because 40GB does not equal 40GB. A kilobyte is 1024 bytes. Manufacturers report a kilobyte as 1000 bytes but drive size is measured where a kilobyte equals 1024 bytes. This causes a discrepancy between what the drives says and what the formated size actually is. This is an old discussion. You also have 3 partions (besides the main partition) on the drive to support the appleTV and you lose some space to these.

Glor

The formatted drive should contain 4 partitions with the following functions & capacities:

Partition 1: EFI - 34.0 MB
Partition 2: Recovery - 400.0 MB
Partition 3: OSBoot - 900.0 MB
Partition 4: Media - 36.0 GB

Partitions 1 & 3 are used to boot the OS, 2 contains the factory reset source, & 4 stores media content. Thus, you have 3.xx GB of media of some sort in your Apple TV.