The Difference between ESX and ESXi is that ESX was the classic version having a dedicated Service console embedded with it, however ESXi is the the version of Vsphere without the service Console. Since ESXi is without the service console, therefore its size is very small and its been referred to as having small disk footprint. Vmware with the release of VSphere 5.0 and later has deprecated the classic ESX version so there is no more ESX now available and we have only ESXi now.