When I took a snapshot, the size of the vmdk file of that snapshot wa just 3 GB. How can this small file include all information of the large disk ? (60% of 250 GB used).
Does the state of a VM really include all the disk data at the moment the snapshot was created ?
Snapshots Disks are Delta Disks! 
They only contain the differences between it and its parent. As a reference, have a look at: Understanding virtual machine snapshots in VMware ESXi and ESX